Tests can be very hit and miss, the acid strength may be wrong, the acid can go 'off' over a period of time, you may get a false reading if the silver plate is thick .... the mostreliable method is the 'water displacement' method, a bit cumbersome but reliable. As you can remove the glass from the bottom of the casing, then it will be pssible. Otherwise, as mart says, a jeweller will look at it.
All the best