Good question and you should know if you are satisfying your wife. A gentleman would never ask if she reached orgasm, he will know on his own. Also the term fake orgasm came from female to make the man feel happy that he is a stud in bed. The stydy says, more women fake orgasm than reach one. Some even are so habituated, that making some sound makes it easier.
Now yuur cconception of vaginal muscle. This is just one indication and not even a solid proof. A lot of woman dont do pelvic floor exercise, they still reach orgasm. A lot of woman does the excercise, they can easily make the guy feel that, but thats not orgasm.
Orgasm has number of different ways to express, from looking at her change in body, from her voice and what she does. There are list of symptoms, everyone doesnot show the same way, but some are very common. You can spend sometime on this site, I wrote on this before, take a look.
But there is one sure shot test to know if she reaches orgasm, if you dare to do so. The asshole gives a sure shot indication, when she is about to reach orgasm, the anus will losen up, during orgasm, it will involuntarily tighten and then release the grip once through. I checked number of times, and I got the same result.
I like your intention behind the question. One true fact is, the more you want to please your wife, you will find you are getting much more in return.