Sometimes it's obvious they have come, sometimes it's obvious they are faking it, most times you don't really know for sure.
Faking an orgasm is a common thing for WLs to do.
Sometimes it's to make you feel good about making her come, sometimes it's to make you stop what you are doing because you are making her uncomfortable or hurting her.
Sometimes you are doing an excellent job but the WL doesn’t want to come. It could be because it makes them tired or they are too shy to or they don't know you well enough or they are saving that for their boyfriend or whatever other reason. So she just fakes it instead.
All girls are different.
Some girls can come very easily and quickly, and some need to be totally relaxed and comfortable, or stimulated in a very specific way, or trust you and get to know you first etc
Some girls are confident about coming with a complete stranger, and some are very shy and hold back.
Some girls can come several times and some are totally exhausted after coming once.
These are so many more variables, these are just a few off the top of my head.