This can be complicated because there are some women who are particularly good at faking their orgasms. Some men will usually find the fake orgasms a turn on until they would orgasm themselves. If you have been with this girl for a long time, you may know some signs if a girl has just faked her orgasm and when she has not.
If the woman does not feel any pleasure with having sex with you, this is a sure sign that she has faked all of the orgasms when you two were having sex with each other. This can be a blow to your ego but perhaps you can make some changes with your usual methods so you can pleasure the girl more.
This is quite difficult to determine because some women are very good at faking an orgasm and some men are very difficult to figure out if a woman is faking an orgasm. One way to determine this is by past behavior. If a woman has faked in the past, you may be able to figure it out based on her reaction and behavior when you knew she was faking it in the past. If this is a new partner, it may be more difficult.
There really is no way to determine for sure because you can’t be exact. If it seems that the woman is not truly pleasured during this time, there is a great chance that she is faking it.