So to answer your question tinks. A man can tell and it becomes more obvious the more skilled he becomes.
He can become more observent to her needs yes but a woman can lie. Remember, Meg Ryan's movie?
Here are some of the signs, but of course every girl is different.
Uncontrollable moaning, grasping of the sheets or partners body (nails - ow!), curling of toes, her body shaking uncontrollably
If she wants you to think your God, she can fake this easily, I have.
The girl com pletely stopping all voluntary motion, her face going into deep dreamy pleasure face, dilated pupils (I have seen this) though it isnt always the easiest as most girls cum with their eyes closed... hmm, wonder what they're thinking about :-D
No women that feel embarrased turn their heads, or close their eyes, or if they are bored and faking it... If they are indeed cumming? Then they are thinking about their pleasure....
A gentle sheen of sweat breaking out over the body. lol try faking that.
Generally that's because two bodies are moving in fast motion it's called sweat.....no faking.
Nipples sometimes retract, as does the clitoris, though you'd only notice this according to the position
Yes, expanding of the clitoris suggests arousement but my nipples are errect pretty much all the time.....not according to any position and usually the clitoris needs stimulation and love so are you doing that?
Breathing - very important sign for quiet girls, it can be very shallow and very fast in the build up to orgasm and of course her heart is pounding and on release turns drastically into deeper and slower breaths
Your making this sound like your a player, studying the art of seduction? Shy girls verses non-insecure "girls"... girls means young.... Women are experienced and know what they want.... But breathes can also be faced..... A woman wouldn't think of doing so... a girl would if she didn't want you to feel bad.
Complete relaxation of her body
Falling of the bed almost relaxation? Yes ... that I agree with.
A series of involuntary strong vaginal contractions (that are not for the mans benefit) that can leave marks on the penis or squish fingers, nice
Your not talking cumming, cumming is plain simple...asm is what you are talking about....
Some girls squirt too
Ejaculate and it's only women whom feel totally comfortable with themselves in my opinon...but yes we can.
So there you have some ways us men tell if you're really cuming. Now, how can you girls tell when we cum???? lol
And, there you have some definate ways of women faking it as well.. The only one you can rely on is when you can feel it, on your penis, fingers, .....
And as for your last comment? Not rocket science
I asked my current girl once what she feels when she cums, and she gave me the most beautiful answer, one I'll never forget
She said, "it's like fire, in my mind, and there's only you."