The biggest "clue" is her flushing. I have yet to meet someone that can do that voluntarily.
Other involuntary indicators are the spasming of her vaginal muscles (my favorite), the legs shaking, crying, and picking up the phone and calling her best friend. Ok, that last one's bullshit, but you get my point.
Other indicators that can be faked: Screaming, back arching, holding breath, fainting, aftershock tremors.
I guess one thing that needs to be mentioned are the indicators that happen that shouldn't happen if she is faking and that is... silence.
Seems like she's just shy and quiet. Congratulations, she had an orgasm!