Maybe he doesn't have one. He could have just met that Hatsune(was that the victims name?) in a bus or train and while talking mentioned that he is a detective. Hatsune has told then that she needs someone who could keep an eye on her upcoming-husband and asks Toru to do it.
Of course that is kinda wild idea but I can't imagine him to sit in an agency like Kogoro does and wait for customers...
And why there even are so many detectives? I mean Kogoro and those people in DC movie X (Private eyes Requiem). Then we have a group of hing-school detectives: Shinichi, Heiji, Hakuba, Natsumi(okay, she is 20 in real), Junya(Natsumi killed him) and Sera. +Detective Boys.