When you're on top of a girl and she's moaning, groaning and thrusting her hips to the rhythm of your body, and she's probably giving the best moaner in the entire world a run for her money, that's the time you'll know she's in her zone and you've made that happen.
Now although in your head you know she's enjoying it and she's having hell of a good time, you can't help thinking, what if she's just tricking me into believing she's enjoying it so she won't disappoint, or just the mere thought of her faking it because you're that bad in bed can send you into a downward spiral of sexual urges, so before you lose it and you start thinking these silly thoughts, I suggest you reinforce your awesome skills in bed with a couple of tricks and twists, and you're never miss an orgasm with her again:
Strengthen your musk - Smell is perhaps one of a woman's downfall when it comes to sex. Women in general, even those strong-headed all services escorts you couldn't get to sleep with you, would fall for you the minute they smell that manly musk on you. And yes, sometimes you'll need the help of a cologne, sometimes you just have to hit the gym more often to bring out that natural man scent, but whatever it is you need to do or chose to do to get her to scream for more, keep doing it because you'll never know when she'll have her next "Meg Ryan Orgasm" with you.
Keep her feet warm - every man knows, and if you don't you're not a man, that a woman loves keeping her feet in between a man's legs, especially in bed. It's their way to caressing you and giving you the signal that she's up for some nightcap and she's working her way up from the feet. Now if you still can't take the hint, I suggest you try sleeping with more women around and try to notice what they do when they hit the sack, if what your girlfriend does is the same as what the all services escorts you've slept with do, then you'd be grateful I gave you this tip.
Take her on an orgasm overdrive - if you're getting all down and dirty, grinding and sweating all together, and you can feel that she's really in it and not faking it, then that's your cue to put her into an overdrive. Touch her body like a fragile box filled with glass figurines while pounding her so hard she's about to burst her bubble, the sensation of a soft touch against the rough friction of your body pressed onto hers will give her two different orgasms and she'll never be able to fake it nor control it.