A girls orgasm consists of a tightening then loosening of the vaginal muscles, so you'll feel it get really tight, then aaaaall loose. some girls moan, some hold their breath, some make animal sounds! (believe me. that is not the sound you think youre going to hear. youre expecting "ooooh, aaaaah," and you get "HEEHAW!!!HOOOOOOOWWWWWLLL!!!")
You can tell by there sounds as they reach a climax ... some will shudder a little bit other wise you will feel them get really wet.. and thier pussy gets loose and slippery
they will probably start getting into it when its coming...
its pretty much the same for guys - how to girls know when guys cum?
when they start pounding her hard pretty much.