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.