I want a clean girl friend how do i know if she is ...

Clean girlfriend? Are you referring to a virgin? Because if you are meaning a virgin, you're also implying that ladies who are not virgins are dirty and I don't think you mean to be insulting a large percentage of the community.

If you are looking for a virgin, the best place to find one is in a nunnery.

If you mean clean of STD's you can use condoms or ask her to be tested (something I would do after you've been together for a while). If you mean hygiene wise you need to be with her for a little bit before you can establish that. It won't be hard to find out once you get intimate.