Not sure what you’re asking. Are you asking to ask her out or if she’s interested in you? We can’t tell you for sure since she’s not there. It could be either. And I would say to ask her out, but that doesn’t do much for your situation. Your mind is saying don’t because of rejection (probably) which is reasonable. But you’re also self-aware that we may be picking up on signals that may not be signals but her just being “nice” and providing “good customer service.”
Ultimately you’re going to have to take a risk or not and let her go.
I would go with something like this: “So there’s this really cute girl [describe her] that works at ______ [place she works at]. It’s hard for us guys not being mind readers and all.” (got to have confidence and a sly flirtatious coy look and tone for it to work). There’s always a corny pick up lines. (you know what vegetable you’d be? a cute-cumber)