It has been estimated by various studies that 70% of women fake orgasms at some point in their sexual lives. Some fake it very regularly -- as in every time -- while others  only do it sometimes. 

The psychology behind faking it is simple: She isnt going to have an orgasm this time, and she knows it. She cant be bothered with a) you trying hard to satisfy her and prolonging the encounter, and b) you feeling bad because you couldnt satisfy her come hell or high water. 

If you want to know if you are being duped, use the following signs that she really is having an orgasm to distinguish the faux from the bona fide, and catch her in the act.

Retraction of the clitoral head

This occurs just before orgasm and provides you with a clitoris-sized hint. When the clit disappears, youre on the right track, so dont stop. This coveted disappearance of the clitoris isnt only visible if you have the lights on and your face all up in it; its something you can feel as well. So, get yourselves into a position where you can easily rub her clitoris during sex, and use it to your advantage. Under the guise of giving her some extra special treatment, you can feel whether shes actually close to climaxing or just faking. 

Increased breathing and heart rate

Listen to the sound of her breathing in your ear: When it starts to get heavier, youll know youre onto something. The change will occur reasonably swiftly when she is about to orgasm, and will be accompanied by a completely unconscious change in the tension, rhythm and pace of her other body movements. Liars and the truthful alike might cl**tch at you and moan and groan, but her breathing is the missing link. Her thumping heart will also be a sign that its for real; if she isnt actually excited, her heart rate and bodily manner will be very ordinary. 

Dilated pupils

Dilated pupils are another surefire sign that your girl is reaching her peak. This might be hard to tell in the dark, but if the lights are on, all you have to do is ask her to look into your eyes as she climaxes. Chances are shell be more than willing to oblige to that romantic request. Just make sure to make note of what her pupils looked like at the beginning of your romp so you can compare their difference in size at the end.

Red lips

This means both sets: Her mouth lips will go a little redder (lips swell and redden upon arousal), as will her vaginal lips. While you are penetrating one set of lips, try to feel the swelling in the other set of lips that youre kissing. Be careful though: Kissing will also increase the redness of her lips, so be gentle with pecks so that you can get an accurate reading of her response level.

Vaginal muscle spasms

Muscle spasms are the biggest indicators of all because she cannot fake or hide this. During orgasm, she will have between 3 and 10 vaginal and cervical contractions, the first few being the strongest. They will grip your penis, and the tightening feeling you experience will be impossible to ignore. 

Sudden perspiration

Hot, sweaty sex? Yes, please. Breaking out in a sweat means her body is at a high level of tension and her muscles are working overtime -- her breathing, heart rate and *lo** pressure are all up. So, once you both collapse into a sweaty heap afterward, youll know shes been pleased.

other important signs

• She may arch her back and feet, have uncontrolled twitches in her limbs (during and after orgasm), and have uncontrolled facial expressions (aka her orgasm face).
• Check her cheeks and neck to see if they are flushed.
• Of particular importance is what she does immediately after sex: Does she leap right out of bed and get on with her day? Or does she go to the washroom and lock the door? The chemical effect of an orgasm on the body brings a dazed look to the eyes and she may just want to laze for a few minutes to enjoy it. If she is missing this post-coital bliss, be suspicious.  
• Her nipples will also harden; it has been estimated that there is a 95% chance that if her nipples arent hard, she didnt orgasm.
• Her clitoris gets very sensitive right after orgasm, so if she doesnt slow down or stop completely after her orgasm, she is probably a faker.
• Making coherent sentences far too loudly is also a clue to her (dis)honesty; she shouldnt really be able to say anything much other than Oh my God! if the job is getting done to standard. Anything remotely porn-like is highly suspicious (loud over-exaggerated moaning, etc.) because porn sex is not real sex.