In my personal experience, it is much more subtle and complicated than this. Some women do show it but you have to be very aware and sensitive to their reactions. Some women become very warm and flush, others started to shake, and some, well, it is easy to notice since they become very vocal or have other visual or audio reactions. Others, as you say, do not show it and unless they tell you, you will not know.
I have told this story before on this forum, I am sure. I was with a San Diego provider and after a nice long ride in perrito, she started to yell obscenities and then yelled, "God damnit, you made me cum!" I was a bit taken aback and thought, "Well, a simple 'Thank you' would have been more appropriate, don't you think?" but kept quiet and attended to my own affairs. Sampson then made a cogent comment regarding my post. I can not quite remember what he actually said but the gist of it was that many chicas feel that if they don't orgasm, then they are not really partaking in sex.