How do you know those glasses on the counter touting 8 layers actually has those 8 layers?

It kind of sounds like all you can really do is see if the glasses have the features you requested. Do they transition? Does it seem like they transition as much as you were wanting? Do they seem like they don't have bad glare? Does it attenuate the glare as much as you were wanting? If not, then return them. This is entirely possible with online glasses retailers, they usually have decently generous return policies.

People don't buy glasses because they have 8 layers instead of 7. They buy them because those layers give additional features and it's the features they want. If they can do the features you want in 7 layers instead of 8, does it really matter if it's only 7 layers?