Are you only referring to 05-06 The Cup in this regard? Because if not...
I focus on a very narrow sliver of the hobby. I have studied 05-06 Cup due to my interest and love for this set.
I have seen multiple examples of 05-06 Cup cards sent to PSA and BGS rejected with a "trimmed" result when I know these have been freshly pulles. Also, there are issues for grading the CONDITION for memorabilia cards thicker than 35pt. It is a much higher degree of difficulty to alter a card's CONDITION with corners, 8 edges and it being three dimensional.
My rule of thumb (far from foolproof) is that for cards thicker than 35pt, MEMORABILIA are far more likely to be altered than CONDITION. For cards 35pt or thinner, many of these don't have memorabilia and scammers would alter CONDITION for immoral lucrative schemes.
There is no perfect system. The largest 05-06 Cup collectors are well aware that some of their "sickest" patches may be forgeries. Smart guys, like ToddP, buy from big breakers directly right at product launch to get peace of mind with lowest chance of acquiring a fake.
With 05-06 Cup, "sick" patches were rare compared to other later years' releases. I remember pulling two best patches I've ever pulled for a W.Wolski ARP and F.Mahovlich Signature Patches that if I saw them on eBay today, I would stay far away from them and assumed they were fake. Again, no perfect system. Buyer beware. Suggest focusing on a narrow sliver and be an "expert" to minimize chances of buying an altered card.
I know my take is sad for me to contend with the horrible reality of forgeries and altered cards in the hobby. I used to love chasing sick patches, but now I've had to change my focus because of these realities. To each their own, but I have zero tolerance for fakes in my PC.