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Ask her if she'd like to go out for coffee sometime lol
Lolz. Oh boy.
This may not be solid game according to the pros, but it's something you can try especially if you don't want to just outright ask her out. You can guide the conversation so that it will make this statement relevant in context, but basically say "I bet your husband would hate to hear you say that." Or really anything along the lines of "I bet your husband..."
Clearly, she won't be married because you've already checked for a ring. Naturally, she'll say "I'm not married", or better "I'm single". If she says not married then say, "Er....your boyfriend rather...." then see what she says there. She'll hopefully say I don't have a boyfriend. Then you can compliment her and tell her "Why not, you're a pretty girl." A compliment in that context would probably be quite flattering, especially to a lowly barista (no offense).
This works two-fold. It lets you know if she is single or not, which is crucial. And it also allows you to test the waters a bit and see if you get some ioi's from her. If she smiles big and is like "I don't HAVE a boyfriend" and flutters her eyes at you, then that's a greenlight. If she clams up as soon as you mention husband or boyfriend and doesn't seem responsive, that's obviously not going to be a good sign. Either way, you can feel her out (so to speak) by making that little innocent comment.