There is no "sure-fire" way of knowing if a girl had an orgasm. It differs from person to person. Does SHE know when she's had an orgasm? Sometimes women haven't played with themselves enough to understand how it all works. If this is the case, she may need some time to explore herself.
The only way I'd recommend to you (instead of trying to use physical signs like these) is to build a trusting, communicative relationship with your partner and talk about these issues. If she's not telling you when she has an orgasm, then there seems to be something missing here... but that's just my opinion.
That said, for me as an individual, I know when I've had an orgasm because of the contractions in my vagina and pelvic area. I do tend to get more wet, but usually I'm wet the whole time so that's not a good indicator. I also crunch up my legs and turn on to my side as the orgasm hits me, squeezing my thighs (usually onto my boyfriend's head).
He has told me that sometimes the taste of my fluid changes just before I come, which is interesting (but not a constant). Go figure.