Increased breathing and heart rate
Listen to the sound of her breathing in your ear: When it starts to get heavier, you'll know you're 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 clutch at you and moan and groan, but her breathing is the missing link. Her thumping heart will also be a sign that it's for real; if she isn't actually excited, her heart rate and bodily manner will be very ordinary.
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 she'll 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.
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 you're 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.