Just remember, anyone in the service industry could also be working you for a bigger tip. The pretty waitress or bartender could definitely think you're cute or find you interesting... or they could know if they flirt a little bit it means a bigger tip.
In my experience having worked in a restaurant.. try to be a regular, just don't be creepy about it. Come back, ask to be in her section, see if she's excited to see you, then just talk to her some. If you think she's interested, just ask if she wants to get some coffee or drinks when she gets off. If you're too nervous, you could always leave her a note, but have some confidence and just ask her.