Apparently, most women do it. Here's how to know when....
According to sex expert Tammy Nelson, M.D., 70–80% of women do fake orgasms (though how could anyone conduct an accurate study if the control group is composed of orgasm-faking liars?!). Anyway, apparently you have a right to be concerned. So many women fake it, says Nelson, “because they’re afraid or don’t know how to tell their partners what they need to achieve a real orgasm. Most women need direct clitoral stimulation for anywhere from seven to 45 minutes, and unless she gives you hints—sighs, moans, ‘Ooh, yes!’—you won’t know if you’re even in the ballpark.”
But there’s a way you may be able to tell if you’re close, Nelson says: “When a woman’s aroused, blood engorges her vulva, her vaginal lips swell, she lubricates, and when she orgasms her vagina pulsates.”
That being said, you could also look to her personality: Is she very accommodating in general? A people pleaser? Are her eyelashes real? If she fakes a lot, that may be a clue.
And if you still can’t tell, you can always just ask her.