How do I know if my girlfriend is realy having an orgasm?

The only way you can find out is while you are penetrating her you will feel her muscles contracting, and her anus as if she wants to suck all of you in. Also, a women will usually scream only while she has an orgasm. NOT from beginning through the end. I know women who fake their screams from the moment a guy inserts his penis. It's hysterical because it's a great sensation but it's not for a woman to scream like some crazy lunatic. Trying to be a porn star! LOL

Well if she is faking it, then she doesn't deserve real orgasms anyway. :)

Okay, there's a few signs to look out for.
1. Pupils dilate. This is impossible to fake.
2. Vaginal muscles suddenly contracting. Possible to fake if she has exercised these muscles before, or knows how to.
3. Face contorts. Doesn't always happen, but if it does, that'll help.
4. Breathing and heart rate sky-rocket. Harder to see, but there you go.

Okay, that should help you. These things will all happen at the same time.

When she is having an orgasm, place one finger on her anus, you should feel the contractions of her sphincter indicating an orgasm.

Who cares? As long as you are having fun then it shouldn't matter if your girlfriend isn't in to what you are doing to her. Oh yeah and for your info playing with her clit for 10 to 20 minutes guarantees an orgasm while you are penetrating her. When her tits turn hard, her chest becomes red and she becomes sweaty then you'll know she is having an orgasm. Never stop even if you are tired, just switch positions and have her ride you so that you can take a break.