I used MangaFox to get all of this information, so some translations (and thus information) might not be reliable. Sorry!
I find it funny that Takagi sometimes say something that becomes a hint and helps Conan realize something (i.e. post box, crossing a tightrope, etc.). I'll clean this up later. :D
Wataru Takagi timeline
- Volume 18, File 171 (It Must Be Similar…): Detective Wataru Takagi met Conan Edogawa for the first time. He also witnessed "The Sleeping Kogorou" for the first time.
- Volume 19, File 212 (Uneasiness): Takagi saw Conan with Heiji for the first time, poking their noses on the Wedding Murder case. However, Heiji did the deduction show by himself in front of everyone without Conan's input.
- In the anime, Takagi saw Conan with Heiji for the first time on Episode 77-78, Distinguished Family's Consecutive Accidental Death case.
- Volume 34, File 342 (The Flying Neighbor): This is the next case where Takagi saw Conan with Heiji. This time, Conan had some input, or something to say about the case. Note that this was after the Desperate Revival arc (Volume 26, File 256), where Takagi saw Shinichi and Heiji together.
- Volume 20, File 198, page 13-14 (Tangible Proof): Takagi had some information about the Seal Bathroom case, but he decided not to tell this to Inspector Megure because he thought it wasn't important. Conan asked him what it was. After hearing about the information, Conan gave him advice—that he should refer it to the inspector because it might help out the case later on.
- Volume 21, File 205 (Midair Sealed Chamber): A year before Shinichi Kudou became Conan Edogawa, Takagi met Shinichi during the flight from Tokyo to New York, which happened to be Shinichi's first case.
- Volume 21, File 209, page 3 (The Inspector's Investigation): Conan did a deduction show in front of Takagi and Detective Miwako Satou for the first time (Treadmill Murder case). Later, Satou commented that Conan was pretty good and told him that he should become a police officer in the future.
- Volume 24, File 232 (Finding Evidence): Takagi did an investigation with Conan (and the Detective Boys) for the first time to solve the Innocent Suspect case.
- Note: Satou believed that Takagi will be able to solve the case because Conan was with him.
- Volume 26, File 256, page 16 (Concealed Truth): Takagi saw Shinichi for the second time during the School Play case. This was after Shinichi's encounter with Black Organization agents.
- Volume 26, File 260, page 2 (The Memorable Place): Takagi saw Shinichi experiencing spasms for the first time.
- Volume 27, File 272 (Game Over): Takagi witnessed "The Deduction Queen Sonoko" for the first time.
- Volume 29, File 285, page 7 (The Suspicion): Takagi started to discuss case information with Conan without reservation—Satou did so as well—believing that Conan had the capability of discovering things that would be helpful for the case.
- Volume 30, File 298, pages 6-7 and 12 (The Time Trap): Satou witnessed "The Sleeping Kogorou" for the first time. She was very inquisitive that she watched Kogorou very closely and then pinched his cheeks to make sure whether or not he was actually sleeping during his deduction. Later in the file, when the murderer's identity was uncovered, Conan prepared his power-enhancing kick shoes to neutralize the culprit. But then Takagi intercepted and did the job. Did Takagi saw Conan preparing for the attack? We can only speculate.
- Note: I don't know whether we should put this on the list or not; I did, just in case. Considering Takagi was in the scene when Satou started pinching Kogorou, he most likely saw it. He might be able to use this information later on in the story.
- Volume 32, File 327, page 4 and 7 (A Stupid Plan): Conan was explaining about UK patrol cars in the 60s and said something about "James' (Black) generation." Takagi then wondered in what generation Conan came from, or probably the decade he was born in. Clearly, Takagi suspected Conan's intelligence or was astonished by it. Later, Takagi and Satou learned about the badges and its functions—well, maybe not the function in which it acts like a homing device.
- Volume 33, File 329, page 9 to 16 (Satou's Odds): Conan called Takagi as Shinichi per "Conan's request". Later, Takagi questioned Shinichi's involvement in the case. But then, Ran called out for Conan.
- Did Takagi hear Ran? Again, one can only speculate.
- I'm also wondering, did Conan already have two phones at this moment? When did he start carrying two phones? Oops... it should be after Volume 47, File 484. Also, Takagi referred to Shinichi as Kudo-kun.
- Volume 33, File 336, page 15 (Clean Scent): Takagi commented that Conan had a sharp mind but act childish sometimes.
- Volume 34, File 345, page 8 (The Suspicious Supporters): Takagi directly asked Conan about his intelligence and how did Conan figure out about detaining certain passengers of the train during the Soccer Supporter case. Conan said that Professor Agasa had ordered him to do so.
- Volume 36, File 372, page 13 (Inerasable Memory): Takagi asked who Conan really was. Conan replied that he'd tell him "on the other side."
And I'm waiting for the continuation of this story. PFFFT.
- Volume 40, File 406, pages 13-14 (Misguided but Successful Arrest): Conan used his power-enhancing kick shoes and ball-dispensing belt around Takagi to apprehend the drug dealer. However, I don't think Takagi saw it because he was running in front of Conan. I guess I just want to point this out.
- Volume 43, File 437, page 10 (Ai Haibara's Resolution): In the previous file, Conan asked Takagi for specific things that would help them solve the Murderer's Car case. Takagi was confused but went ahead and fulfilled those requests. Then, in this file, Takagi saw how those things helped in solving the case. He later commented (in his mind) that Conan had foreseen what would eventually happen and acted upon it.
Volume 50, File 516, pages 12-13 (Message of the True Criminal): Takagi called the suspects one by one and had let Conan to listen with him. I guess this is not really important, just pointing it out.
- Volume 54, File 555, page 6 (A Dreaming Star): Takagi commented on Kogorou's habit of changing his opinions about the case whenever he does his deduction shows.
- Volume 60, File 627, page 4 (The Hammer Ma's True Identity): Apparently, there is a difference between the poses of "The Sleeping Kogorou" and "The Deduction Queen Sonoko," and Takagi knew about it.
- Volume 60, File 630, page 3 (The Bitter Truth): Inspector Megure and Takagi were stumped about the Poisonous Coffee case and couldn't find any more clues that would help them solve it. During this conversation, Inspector Megure then pointed out that at that point of the investigation, Kogorou should have transformed into "The Sleeping Kogorou." Takagi agreed, but then he added that before it could happen, Conan sometimes should say something unrelated to the case, which would usually somehow become a clue for the case.
- Volume 62, File 642, page 15 (False Friendship): Satou received a text message from Conan containing the real identity of the culprit and the location where he would be with Azusa and her brother. In the text, Conan wrote that he figured the case out "by coincidence."
- Note: I'm not sure whether this would affect Takagi's "suspicions." But since he have a relationship with Satou and that there were instances when they talk to each other about the cases (and Conan's intelligence), I made note of this.
- Volume 63, File 653 (My Deduction): Takagi saw Heiji and Shinichi working together on the Highway Murder case for the first time. Takagi did saw both of them during the School Play case but they were working separately.
- Volume 63, File 654, pages 3 and 12 (Deduction Answer): Takagi witnessed Shinichi experiencing spasms... again.
- Volume 63, File 659, page 3 (801, Shakuhachi, and First Class): Takagi was commenting on how good the response of Nichiuri TV staff was after the accident of the head judge of "National Kojima-san Competition," only to be informed that it was all Conan's doing.
- Note: For me, this case reminds me of Shinichi's first case on the airplane, where he first met Takagi as well. How? Well, in the airplane case, Shinichi had done an investigation (with the help of the airplane staff) even before Inspector Megure and Takagi came in the crime scene. In this case, Conan had done the same thing, this time, with the help of the TV company staff.
- Same volume and file, page 7-8: Takagi asked the three suspects questions regarding the competition per Conan's request. Takagi really trusts Conan's judgment when it comes to solving a case.
Takagi also learned that Conan is a Sherlock Holmes fan. Well actually, Conan quoted Holmes. It's not really a strong evidence to say that Conan is a Holmes fan.
- Volume 63, File 660, page 7 (Truth about the Contest): Takagi learned that Conan had asked help from Shinichi to solve the Kojima Contest case for the first time.
- Volume 64, File 671, page 10 (The Connection): Takagi let Conan accompany him in questioning the relatives of the victims of the Whistling Man, while the other members of Detective Boys waited in his car.
- Volume 72, File 761, page 14 (The Truth of the Karuta Cards): Conan used his tranquilizer watch, ball-dispensing belt, and power-enhancing kick shoes in front of Takagi. Actually, I'm not sure whether he saw the tranquilizer watch in front of Takagi since it wasn't show in the panels.
- I think Conan used the ball-dispensing belt in front of Takagi. He actually told him about it during the Jewel Robber's Suicide case.
- Volume 73, File 770, page (Sera's Careless Deduction): Takagi referred to Kudou Shinichi as "Shinichi-kun" for the first time. Conan called Shinichi to help out with the case. This is probably the second (or third?) time Takagi learned that Conan ask helped from Shinichi to solve a case. Conan did this again in Volume 74, Files 778-780 (East Vs. West case).
Volume 73, File 793, page 15 (Private Eye): Takagi met a Black Organization agent (though he didn't know about it at the time). Hehehe. Oh wait, there was Vermouth, but she was disguised as Dr. Araide. I guess, to be more specific, he met a Black Organization agent who was not in disguise.
Gadgets that Takagi (probably) knows about
- Watch (flashlight and tranquilizer functions)
- Badge (walkie-talkie function, probably not the transmitter function)
- Ball-dispensing belt
- Power-enhancing kick shoes