It's tough to tell in the old (non-edgeview) holders. Counterfeiters have put, for instance, a milled-down obverse and milled-down reverse of two different coins to "create" key dates. The coins are genuine in this case, just defaced and put in fake slabs, and with no way to examine the edge, you would never know.
My point is that it would be more beneficial to individuals to study the characteristics of counterfeit coins than of counterfeit slabs.
I still don't follow. If you can ID the COIN as counterfeit or genuine, why should you need the slab?
Your real coin, could be in a fake slab. Your fake coin, which you have not identified can be in a fake slab, but yes, you'd need to know how to tell the difference between real and fake coins, as well.
And if your real coin is in a fake slab, why should you care? Particularly if you're buying the coin and not the slab!
Well, do you want to show us fellow collectors your real coin, in a fake slab? I'd want everything I own to be authentic