What’s a nice way to ask a women you met on a dating site, whats she on there for ?

No need to ask. Deliver a compliment on how she appears to good to be on the site. It will open the line of communication up and you might get her to offer some information. Asking the question leaves room to come across sounding pretty dumb. You never know how people take certain things when reading a message when you don't have tone of voice or body language to also read off of.

straight up ask. don't need to be proper. it's just a question.

Hey ya be'yatch, ya lookin' for the "D"?

"What's a nice woman like you doing in a place like this?"