One good way to get a Grohl auto is to buy a signed copy of his book. They were sold in the Barnes & Noble holiday sale the year issued. That's how I got mine. His sig is way too simple for a forger otherwise.
It's a minefield, for sure. I have been burned more than once on a bad rock auto. Now I am very careful. My best gamble ever were these two I fished out of a bargain bin at a paper fair:
Both passed PSADNA.