showaddywaddy:
When you take a watch apart you will see that (typically) every jewel you see on the top plate has a mate on the bottom plate. The balance staff has a double jewel affair with a hole jewel and a cap jewel, so you would have two on top and two on the bottom. I’m sure there are exceptions to this, but as a general rule you could count them that way.
Caveat: Others here know much more than me – I’ve only worked on very old pocket watches. 
Thanks!
Tom