Is my girlfriend a virgin?

What difference does it make?  The vagina is a wonderful thing.  A baby can stretch it into the size of a manhole and a few weeks it get tight.  If she is giving it up to you just be happy you are together and don't worry about history.

if you yourself don't already know the answer to that question, than you should first be TALKING about it.

Ask her

I don't think you're getting the answer you want here. Maybe if you put your reason to know…

If your doubt is coming from some kind of male pride of being the first, it's important to speak to her first, like many have said here. She probably would want to tell you how not important it is. But if you're full with mistrust to her, you maybe need to make some questions to yourself before.

Now, if it comes up that she doesn't even know, cause some kind of fear that she might've been violated unconscious or something like that (btw, very unlikely to happen), and you wanna help her find out, you have all the answers here before mine, or maybe you need to go to a gynecologist.

It'd be really nice to help you out.

Yo - you really need to talk to her about it.

Aside from all that's been said here - you should be able to tell if she's shy, isn't sure about what to do, keeps saying no - but that's not 100%. Best to talk to her and ask her.

And what diff does it make ? 

You can't tell. Chances are, her hymen was broke the first time she used a tampon. It can also get broken if she does horse riding or jumping or vigourous activities that are similar.