The most common fake MOP,Etc, are Jay Scott and they will be so marked on the back.
Real MOP will always be colored like Mother of Pearl (Abalone shell) and will have some iridescence. Real MOP will never have any form of backing!
Once you have identified real MOP all you will ever need to do is look at them and you will know.
Or you can tap them gently with a light metal object like a pocket knife blade. Ream MOP will give a distinct "clink", like stone. Fake will sound like, well, plastic.