I have found out that the numbers stamped on the inserts are not an exact match at the moment so reproductions can still be indentfied.
I have also found myself and from other threads I have read over time that the smell and feel is different to modern used cardboard. Also you should question the seller if you can and get any past history he has of the inserts.
Depending on the vehicle/s we have a great vehicles and playset section that contains inserts to compare with, it is the first link.
I hope this helps