Guys- signs that you like a girl?

i look at her constantly in the eyes, sometimes with a small smile and sometimes with a serious face

Wow great question !

It obviously always depends on many factors, but I believe if a guy really likes you he will do the following:

1- takes time to talk to you
2- look into your eyes if he isn't shy
3- polite/funny/nice/touchy feely/teases you/compliments you
4- hint that it be nice to do stuff together
5- goes out of his way to be nice

Also use your intuition, every situation is different, there are other obvious things like him telling you directly that he likes you lol :)

Cheers !