1. He will avoid seeing you
If a guy isn't into you, he won't make an effort to spend time with you and get to know you. Simply put he will not go the distance.
2. He doesn't call/text you
This one is simple. If you don't hear from him, it's because he really has other things on his mind and they are most certainly not you.
3. He doesn't get jealous/care when other guys talk/flirt with you
Any guy that likes a girl will try to assert his dominance...if a guy is flirting with you and he could really care less, then this says a lot.