It's funny that this question can now be earnestly asked, incl. by PSA fans

[drcy]

An experienced eye, hobby knowledge, logic, common sense and the input of friends make up something currently more reliable at identifying altered raw cards than a PSA label. If one needs evidence, mere common sense points to most high-grade Pre-War cards in PSA holders likely being altered. Yet PSA has labeled them all as unaltered.

Granted, PSA is the particular target of sneaky altered cards (though much of their own making--formation of PSA registry, marketing, apparently allowing known alterers to continue to submit cards, etc.), but that's a reason (or excuse for) why the facts are the facts not a refutation of the facts.