How do you show affection without smothering somebody? How can you tell when you're becoming too needy?

Stop worrying. Your actions would tell how much you miss her like pursuing to meet her even for few minutes just to see her and tell her how you feel at the moment. And be a gentleman, like you can ask her if you can hold her hand, and when she says yes then do it, in that way you would avoid being annoying because it's with her permission.

She will let you know if you are becoming too needy. If you feel her pulling away from you and you have not gotten into any arguments then you may have something to worry about.

I would just talk to her about it.If your worried about being needy then tell her that. If you don't say "hey" often she might feel that your not really all that interested. If she's a shy girl she probably doesn't want to seem needy either. Chances are she likes you, so just talk to her about what's on your mind.

if you feel that way. just go ahead and tell her. or chat up like" how are u" something like that.
if she feels the same way, then she wouldn't feel needy. maybe she is not sure how you feel about her either. Life is too short for all those waitings, trust me.