I don't think waitresses HAVE to flirt. I've been around enough that are completely indifferent and all that. Sounds romantic what you have going on.
What I would do is write her a note, and find a way to get it to her. Slip it in the bill when some comes and gets it, or leave it with her tip.
I know you think that's bullsh*t or lame... but it totally depends how you word it. That's all there is to it.
"I don't mean to embarrass you, but I think you're lovely. Sometimes my thoughts linger on your smile, and it makes me smile. May I buy you coffee sometime? My name is _____. If I'm wrong just blush. :) I had to say hello". And if you can pull it off add a funny teaser at the end.
I mean... think of the alternatives. Will you ask her out and maybe embarrass her in front of all her co-workers? Follow her to her car like a creep? Post in 'missed connections'. ...
LOL, Well, a good line I remember is, "Can I get some coffee?" and just as she's about to go and get it, you stop her and say, "No, no, I meant coffee with you and somewhere else. Would you like to go get some coffee when you get off?" There's nothing wrong with asking a girl out. You just gotta ask, and accept if she's got a boyfriend or something else going on if you get shot down. Just go with the flow. Good luck man.
Well I don't think that there is a better , faster and surer way than asking for her number and letting her know that you would like to know her better , that will save you from the trouble of wondering what's she is thinking and will give her the hint that you are interested in her .