Questions about fake slabs

[pbspelly]

I was following the Buyer Beware thread and had a few newbie-type questions:

Are there a lot of fake slabs out there?

If the slab is fake, what does it usually mean about the card itself? Completely fake? Or more likely trimmed or rebacked?

Is it primarily high value cards (such as the Rose rookie) that will be fake slabbed, or do a lot of lower value cards get fake slabbed?

Is there a way to check with the grading companies about a slab's authenticity before buying something?

I've seen comments about people cracking open a slab. Why would someone do that?

(BTW, before someone recommends that I search the board, I've done a lot of looking but didn't see the answer to these questions)