Look at her left hand, second to last finger and see if she's wearing a band, if not, then I think it's fair to ask. If she is, you could ask her what she did last weekend (if it's a monday) and if she says that she and her husband went somewhere, then you know. If it's Thursday or Friday you could ask her if she's got any big plans for the weekend and see if she mentions a significant other.
Honestly, she probably would be flattered if you asked her outright. I've had men straightforwardly proposition me even when they know full well I'm married and honestly, I don't begrudge them anything. It's flattering just the same.
The more subtle you are about it though the classier you'll appear to her.