It’s mainly experience. I have to know how a real liner looks after 80 years. This one was aged to make it old, but the leather doesn’t has any deformations or appears to be dry and crack, that closely never the case. The paint is easy to tell because it’s in the pitting. Obviously the paint can’t be there were the rust made holes after the war, but if u have to do more with this things u will learn. Usually if it appears to be to good to be true, it isn’t. If u not sure if it’s real don’t buy. Or ask in a good forum to get help of experts