Most Helpful Guy
Slower response times, wandering eyes, more conservative posture, hanging out with friends at the place of you. Etcetera.
Most Helpful Girl
I don't think that you need a list of signs to define it. IF you feel as if you two are drifting apart and emotionally are distant, then no list is needed here. Believe it or not, you can simply - without saying a word - stop contacting him. Stop showing him you're waiting for him, or wanting to see him or communicate.
Pretty soon he will probably realize you're slipping away. If he comes back and comes closer, that will tell you that he's missed you. However, if he does nothing, then you can safely assume he doesn't want to be close.