I'm looking into buying a pair of full quarters for my 67 RS, the guy says they are NOS. Other than the GM decals, how can i be sure they are real NOS parts? are there any indicators that would tell me the difference? Metal thickness? stamping differences?

 

Date code, read here :
Sheetmetal Date Code

Plus body line should be sharp, later production runs, body line was rounded due to die wearing.
OE qtrs have a couple inches of flat on the body line before 'Coke-bottling out' as aftermarket, there is no flat length - see Post 46 below
Read here also:

'67 AMD quarter panels, tailpanel - Team Camaro Tech

Good luck