In my opinion/experience mostly "contractions", like
@Surveyor wrote.
My ex was veeeeery sexual and came very easily, so I got used to it, so used when she faked, I could tell (at least some of the times she faked, I'm sure lol).
So that's the thing, you will never be a 100% sure, but the signs are mostly unvoluntary. Honestly girls are not thaaaat different from guys, other than having a much harder time cumming on average (my girl now never came in her life, for instance, I'm still trying to get her to cum, I got close few weeks ago). If they get like really tired and maybe start complaning about you still stimulating them (like their clit or pussy getting sore, but without actually being that physically "damaged"), then they probably came. Girls have different ways of cumming as well, sometimes they are very loud, other times quiet, think about how your own orgasms (assuming you have had enough of them) had a whole range of intensity and duration to them, girls also have that, maybe even more so because most don't seem to have much trouble cumming multiple times in a row (other than their body parts ACTUALLY getting tired and sore).
But I guess the girls in the forums must know better hehe
