How can I know if a boy is really interested in me?

I think the reason your question went unanswered for 9 days is because its hard to figure out. I'm just saying, a better answer might come if you can update your question a little.

So he asked you at a party "who are you talking to on your phone? " twice, and also bumped your shoulder. That's what we know, but there are other scenes un-described too, apparently.

Well, based on what you say, he probably doesn't like you. Or at least if those are the most flirtatious, "into you" things he does (and they are, right? Or you would have come up with better examples for your question. ) then he's not into you much. "Who are you talking to? " and "bang. Ouch. Sorry about your shoulder crazy jumping cat woman girl" are not strong signs of love.

But if he flirts, smiles, tries to talk to you, tries to sit by you, tries to touch you (even in obnoxious guy ways like knocking your shoulder), then he might be interested. Especially if he does so AFTER seeing your cat woman dance. Why don't you go and do all that to him (flirt, smile, talk, touch) and see how he reacts? Then you'll know.