You may want to find a girl that is single. There are a few things to keep in mind on that subject - 1) you may get hurt if you date a girl that has a Thai husband AND 2) the girl may tell you she is single when she is in fact not AND 3) both 1 and 2 may be true!

So here is how you ask a Thai girl if she has a boyfriend or husband -

Do you have a boyfriend? Khun mii fan mai krahp?

Do you have a husband? Khun mii samee mai krahp?

Possible answers -

Yes I do. Mii leeo

Not yet - yang mai mii