For me, I feel confident her orgasm is real by at least some of the following indicators:
Rapid vaginal contractions
Hyper sensitive clit immediately following (with most, but not all ladies. Some are multi-orgasmic and can keep going with little, if any refraction period required.)
Squirting (rare, and not always an absolute indicator)
Erect nipples
Flushed face, neck, and v-shape flush down to her breast bone
She offers me a tip instead of expecting one (had to put that in there lol. Actually the only tip I ever received from an mpa was her saying "Look both ways before crossing the street!")