Don't say I never did you any favors...
1. The younger the woman, the more likely she is faking.
2. Most women do NOT orgasm through intercourse alone.
3. A woman's chest area will be flushed if she isn't faking.
4. The walls of her vagina will be swollen and secrete a considerable amount of moisture
5. Breathing, facial expressions, noises, eye movement...all can be faked, and often are.
Why do some women fake it? Because they don't want the guy to feel bad or think he sucks in bed. They start faking so as not to crush the guy's ego, but as time wears on they grow to resent it and feel like sex is a chore. Younger women are usually less experienced and don't know their own bodies or aren't comfortable enough to really relax. They are also more concerned about what other people might think of them, or how they look while having sex. Some fake it just because they want to try to control how they look during sex. Yeah, stupid, I know...but true just the same.