Most Helpful Guy
he never calls or texts you as much as he use too, he may not want to hang out with you like he use too, no or little sexual activity.. there are many things but those to me would stand out the most, also he always is with friends and never/hardly spends time with you.
Most Helpful Girl
If a guy has lost interest in you, you'll know. He stops making plans with you, looks bored when you are together, texts you less or doesn't text you at all, prefers to spend his free time with his friends, doesn't touch you (in both romantic and sexual way)...
I hope this helps!