How do I know if my friend loves me back? If I ask, it'll ruin my friendship with him. If I don't, I'll slowly go insane.

Comments:

If your friend is really close to you im sure he might grab some hints. You can casually bring something that talks about relationships and funnily quote how it'd be if it were the two of you. If he reacts in a good way then you have a really good chance of starting something beautiful. If reaction was like he feels weird about it, give it some time. Repeat, but different stories different strategies. Keep in mind that you just have to know that if they feel the same way.