Guys are simple.
If you are like that relation, you can tease around pretty easily.
the guy's reactions would be...
-Frozen : Fastest way to tell if a guy like you. (Say something random when he make you upset or something like "I like you." But do this only when the guy is feeling very sorry for you.)
-Huh? : They act rather dumber than usual. Some might even hallucinate due to high level or hormones reacting all at once....
-Blush : Another fastest way. They will go red (like me) when they like you a lot.
-Denial : Not always, but, the longer they pause before saying anything like "don't be stupid" or "You're joking" , the more they like you. but this isn't the best way, but if they take really long time to say that, maybe.
-inhale: This is kinda hard to notice, but, guys either *gulp* or very light *Gasp*. This shows that they are shocked... hmm... even I don't know how to explain this.
I'm sure you saw these in DC series. It's pretty accurate if you ask me.