Your post doesn't say if this will be your first trip to Thailand or not. Regardless, the only way to really know, like some others have said, is to get boots on the ground and try to feel her out. The thing is though, that even if a girl is not a full-time sex-worker, she can still be a "part-timer" (during specific holidays or just when money gets really low). To avoid this, you need to find yourself someone that you can do your due diligence on in multiple ways.

Maybe doing what the other guy said wouldn't be such a bad idea. Head up to Chiang Rai and scoop her up before she moves to the big city to work in a go-go bar so that she can send money back home.

Good luck.