It takes experience, many punters are tricked into believing they have given a WL/ML an orgasm or 12. They can fake shaking/shuddering/screaming/etc. Some WLs know that certain punters get a boost to their ego if they think they have given them the big O.
Note if the girl tells you she has an orgasm she could be lying.
The simple answer is: Who cares
Some things to look for: more pussy juice (a squirt is a sure sign); dilating/pulsating vag and starfish; serene smile; genuine connection.
Some things to try: foreplay; foreplay; foreplay.
Don't even bother if there is no connection, however if you have max stamina, a rock hard cock, and great technique sometimes you can break a disconnected WL out of the "don't give a fuck zone" and take her to "ecstasy land" and give her an orgasm. I remember doing this to a disintered WL once and after that she used to run at me when I entered the shop and jump on me.
A while back I was in no doubt as to what happened when I was pumping this WLs vag and she squirted with my cock still pumping her, this was the first squirt for the session, she was quite dehydrated after ...