There's one simple rule, which we European boys learned from Miss Xaviera Hollander: "only if your dick gets wet up to your balls, you can tell your girl orgasmed."
Remember, research shows that 90% of British women (let's say women from the West) frequently fake orgasms (more than 50% of the time). Moreover, neurological research has shown that women are very apt at faking orgasms (this is a skill with many evolutionary advantages, so it's not weird to find it).
Add, finally, that only 1 in 10 people are capable of spotting lies and false orgasms on a women's face, - and it's safe to conclude that your chances of knowing whether a girl has had a real orgasm are very slim.
Hence, the "wet-up-to-your-balls" rule is the only credible guideline. Just asking your girl whetrher she's had an orgasm won't suffice. Looking at her faking it during sex doesn't work either. She will probably lie/fake it in both instances. And you can't see on her face whether she orgasms or lies either, so you will never know.
Luckily, I belong to the category of people who are very good at spotting lies (we've done the test here at LPSG), - which means I can tell whether a girl fakes it -, and I've been so lucky as to have had girlfriends who leaked so heavily that my balls got all wet each time we fckked.
The only thing you can trust (besides the physical proof of her wetness) is a girl who says: look I don't always orgasm when I have sex, but that doesn't mean we can't have fun. I enjoy orgasmless sex with you. Then you don't have to worry.
Finally, remember that not all girls get spontaneously very wet when they orgasm. So it's not a "water-tight" rule either.
