Based on what you said, I'd also say to ask her to dance at the swing dance. The problem is, unless she's obvious (which is probably not likely) you won't know if she likes you or not. This is because a) as you said, she has to say yes if you ask her, and b) dancing with someone or going to a dance with someone doesn't really mean that you like that person. You can kinda judge by how she acts around you, see if she throws any hints... but in the end, these are all just guesses. The only way you'd be able to know is if you asked her... and you can't be ambiguous either. You gotta be straight about it. However, this judging by her hints and actions is a good determinant of whether or not you bring it up.. and when you do.