Sorry if this is a stupid question, but when people refer to a signed crystal, how and where is it signed?
The 'how' is that it's etched into the crystal itself. Will be visible with a loupe or microscope, and if you know what you're looking for, it's possible to see in the right conditions with the naked eye (I just tested with some of mine).
It should be right in the centre, though I do have some watches where it appears slightly off-set, and I've considered that normal.