Obviously the easiest thing to do would be to just talk plainly with whoever it is that you've developed feelings for. Find out if they feel the same or not. But a lot of people aren't comfortable being that direct. So if you'd rather not do that, then I'd suggest the next time you two are spending time together, try to gauge how they seem to be reacting around you. Do they flirt at all? Do they make excuses to touch you? Do they lean in close when you're talking? Those are all good signs that this person is also interested in you.