I could be mistaken, but I haven't seen any fake B-18 blankets in any mass production. I'm sure with technology it could be done, but because of the felt material and the lower prices of these blankets vs. cards, the B-18's haven't been a target for most scammers.
I'm not aware of any of the grading companies which grade these, but you'll want to watch edges for fraying, fading to the whole blanket, stitch holes around the edges (many people back in the day sewed these into larger quilts, so some blankets have been detached from quilts, but now have stitch holes all around the edges), etc.
If you want to see some scans, I have an album of many of the blankets at the link below...
http://s45.photobucket.com/albums/f8...18%20Blankets/