Option 1) Buy from Reputable suppliers
Option 2) Open the case behind and check the movements, just like a Durian. By the way, I do not eat Durian. In the case of the watch, you can close it back but for a Durian, it will stay open forever.
Option 1 has risk of honest mistake by the suppliers, I got burnt a few times but it is part of experience.
Option 2 has risk of rejected or offended by the sellers, if you refused to buy after opening the case. Just like the Durian.
I hope my Options help you in the future.
I just got a package and it is a 1920s Traveller Table Pocket Watch with Solid Silver Case.
As you can see how big compare the width of the Singapore $50.
Hope you learn something.