Female bartenders in particular are always slightly flirty (except when they're extremely flirty) and are very good at subtly bending and posing in ways that highlight their tits and ass. This maximizes tips. I don't blame them for it. I'd do the same in their position. But basically, never ever think a bartender is into you unless she's actively touching your junk. Good way to blow a lot on tips and drinks you don't really want. If you want to hit on the barmaid, do it sober and get ready to get rejected.
At least, that's my experience and that of everyone I've ever talked with about this.
I also consider it inappropriate to hit on someone when they're not in a position of equality, such as when they're at work. I mean, it's fine to flirt a bit, and if they reciprocate, to move on to actually hitting-on, but it wouldn't be right to just open up on a cashier or waitress or whatever.