I saw a red AAR Cuda on E bay 2 weeks ago, the seller said it had original fender tags. Someone corrected him & told him it had repop fender tags. How can you tell by looking at the fender tags that they are repops?
Probably because it looks too good. An original tag will likely have some corrosion, and most are bent because the factory painter bent them up to paint underneath, or so I've read.
A lot of times it has to do with the codes. If you know fender tags you know they follow certain rules about what should and should not appear. The reproducers don't always know those rules so there are giveaways as to the fact that the tag is a reproduction. Post the car in question.