First off don't think of her as a 'treasure' as it's a mindset that says she's more valuable than you are, she's the treasure and you are the poor treasure hunter and that's not a great start.
I'm sure she's lovely but concentrate on your own value - you are the prize to be won, be that having a great idea for a day out, witty jokes, interesting conversation, a knowledge of local history, a fine collection of vintage bicycle clips, whatever you are good at my friend.
The mindset here is... Yes she looks nice but is she worthy of your future attention and time...? You think she might be but are not yet sure as you currently know nothing about her... It's up to her to convince you.
So on to the mechanics, you said you thought she was looking at you - well just bring that up then next time you are in there and see what she says.. "I'm pretty sure you've been trying to catch my eye and it got me wondering about whether we'd enjoy each other's company on a day out..." **remember to smile** "I think we would..." and leave that hanging. No question, no rejection, no embarrassment if she says she has a boyfriend - you stated the truth and you'll be back in for a coffee again the next day.
Oh and have an idea of what if she shows interest. Good luck.