Sadly, there aren't any. I know people say it a lot, but there really aren't any 'signs'. Some people are just really nice or really flirty. The only way to know is to get it straight from him.
Trust me I completely understand the anxiety; I have it myself. If you have any friends they could just walk over with you while you ask or they could ask for you. Also passing notes may seems a little elementary, but it's cute and you could get an answer, that why.
If you want this to turn into a relationship you're going to have to talk to him one on one sooner or later. Why not start with 'Sooo I kinda like you' or however you would ask ^.^