I developed feelings for my straight friend. He says he likes men, but not sexually. I think he might have feelings for me. How would I know?

Asking him is the only way to know for sure. If you feel confident and you don't want to lose the chance to be with him romantically, you can give it a try! I'm sure that, even if he doesn't return your feelings, you won't lose his friendship because of this. Some bonds are too strong to break. Just be aware that, even if he has feelings, you'll have to settle with a purely romantic relationship with him if he's not sexually attracted by men. Consider how you feel about this, and if you think it can work, you can give it a try!