What determines if a burned dvd is fake in an Xbox 360?

I want to know how the Xbox 360 tells if a dvd is fake. It could be thickness of DVD, how its actually burned, or even the color on the backside! I would just like to know so I can LEGALLY burn backups of games I own. Plus, I've heard all about flashing the firmware, so if there's another method of making the Xbox think it's a legitimate DVD in there, then please tell me.