first of all, make it casual (well, as casual as possible in your situation, i know it's a lot easier said than done)
1.) "hey, are you free tomorrow [afternoon, night, etc.]?"
2.) [if she says no-->] "Ah, okay. That's fine, I was just wondering if you wanted to [hang out, catch a movie, etc.]"
(that way, you still tell her what you wanted to do, so she knows for future reference.)
3.) after you say this, if she acts interested or says she wants to do something later, either get her number or give her your number. don't act too excited.
4.) [if she says yes, she's free that night-->] "Awesome. Wanna [hang out, do something, etc.]"
5.) at this point you'll have a good idea whether or not she wants to. if she says yes, then coordinate times. if she doesn't act interested, act like you only asked her because you were bored.
6.) if you guys do end up hanging out, make sure to make the time you spend together casual the first time around.
PS: if she has a boyfriend, you can show your disappointment, but not enough to make her feel bad for... not cheating, i guess. say something like, "oh, okay. sorry, honest mistake." but plan it so that it's at the end of your shift so you can have an excuse to leave before things get really awkward. hope it helps!