Good question! It seems that we may be relying on the word of whoever is doing the selling. One "un-issued" that I bought
had a perfectly un-marred bolt face as well as other bare metal surfaces that did not show any signs of metal to metal contact.
Additionally, most of the outer surfaces of the SKS were un-marred and new looking except for a few storage blemishes including
rust and other signs that it had been stored for a long time. And it came in cosmoline that was dark and dry.
I guess it is a matter of trust and detective work (or self-delusion) on the part of those who care.