I'm a bartender/waitress. I don't ever flirt with my customers, but my smiling and laughing may come off like that. I tend to my customers with a positive attitude and usually with witty jokes, if it's the right crowd.
I can understand that a usual customer might find that because she can remember your "usual" drink, or because she knows you always order an appetizer after your second beverage, it must be an interest... right? Not really.
We like to be efficient and give you a pleasant experience while with us; that's how I am, at least. If you think she is flirting with you for a relationship, then ask her when she gets off one night and feel out her reaction.
Make a move.. That's about the only way you can really tell.. too many people are great at hiding their real intentions anymore.. But if you put the spot-light on her, you'll find out real quick if she's interested or not.
And remember, just cause you are asking her out doesn't mean you're a needy guy. Nor does it mean your sold on who she is. And you should never be bashful for the way you feel. Wear it proud like a real man.
P.s. by making a move - I'm not talking about flirting back with her.. I'm saying something simple like: "I'd like to take you to dinner." and if she responds: "I'm taken" then you can drop it by saying: "That's a terrible shame. But he's a lucky guy".. Or if she responds: "I'd like that".. then you'd know her interest is there, right from the bat.
99.9999999999% of the time, she's flirting with you to get a bigger tip. And yes, I know you'll probably say "but I think this is one of those 0.00000001% times!" It isn't. She's only attracted to your wallet.