Do you authenticate graded slabs?
In short: I can give an opinion on a slabbed card, with the limits stated: the holder blocks some checks, such as weight and the light test. I also look at the slab and label themselves.
Through the plastic I can still check print pattern, font, colour and holo under magnification. Checks that need the bare card are marked as not performed on the sheet.
Fake slabs and altered labels exist, so the holder and label are examined too, and anything that does not match a reference is noted. If a check matters to the conclusion and the slab prevents it, the finding may be inconclusive.