Eyes. The doorway into the soul. As The Eagles knew, if you're lyin', you can't hide it in your eyes. And it's true - they really do say a lot more than slippery words ever can. Does she no longer meet yours? When she talks to you, are they smiling, or glazed over? Or are they flitting about the bar, looking for a better match?
Watch carefully when you talk to her, or ask her a question. There's no need to be a fortune teller - just a simple glance away every time she answers you will be obvious enough. There could be other explanations, like if you live in a hummingbird farm, but the simplest is that she's not telling you the whole truth. The whole truth being that she's found another bed to lie in.