It depends on the guy, but you ask questions or tell him your concerns involving him. If he has developed feelings for you, he will take actions, no matter how small they may seem.
Something just as simple as "Oh, I wish you would call me more often", then he might change a bit and call you more than before. This means he cares about how you feel, meaning he has developed feelings for you.
Just talk to him and see how he's reacting to you.
He may come around more and be more affectionate, sentimental, or romantic. He'll seem to care more about what you think of him and what's going on in your own life. He may seem more committed to you and will probably want to share more time and experiences with you. He might seem a little bit more nervous around you, because he wants to act and speak just right so you don't lose interest.