Thanks! Good to know what to look for. I'd be guessing that refractional properties are nothing to go by, that if there was a difference that it would be too minute to actually make out as I'd not expect tempered glass to be much denser than normal glass. However I'll give that a try too, just out of curiosity, I have a hand held laser and could see if the angle of refraction changes between panes. Not sure if that'd tell me much but, trying doesn't cost much.