But yeah, when I was first dating my girlfriend (second base max) it of course gave me erection while we were making out. I knew that she would freak, though, so my brain kinda didn't allow full-on erections. Perhaps this is where you and this guy are at.
look at there eyes if it looks like something is on there mind or if they are moving there hips to try and make it less noticeable. you can always reach out and brush your hand along there crotch but that's only if you two are close or on a dance floor.
If you are feeling his crotch and you aren't sure...he's pretty small, or he doesn't have one.
Anyway, some dead giveaways are: a reluctance to stand up, or putting something on his lap.
When I was in HS, the girls on the bus used to make a game out of trying to give me an erection. And I had books to put in my lap to cover it with, and they could still tell. It was terribly embarrassing, until I overheard them whispering and I realized they were doing it on purpose.