Okay, take this with a grain of salt since I may not actually be INFp anymore... but, if it were me, things to look for:
- immediately gets this goofy grin when you show up, like instantly
- laughs at most or all of your jokes (doesn't just do the smile and half-laugh thing, and doesn't hold laughs back)
- (once I'm comfortable) touching - light punching, pushing, pulling your arm, etc. and I also kind of tend to let the ESTp have her way with me. In the past, one's played with my ears, jumped me from behind, stuff like that. If he doesn't outright push you away, it's a sign of interest.
Aside from that... I tend to try and convince myself that I don't actually have an interest in someone.