There is no guarantee...but when dealing with a trusted and reputable dealer...if it does turn out to be a fake there it is much easier to get your money back. Plus, these people have dedicated their lives to studying coins and as a result are experts in their field. Can they be fooled...yes, of course. But, the likelihood of buying a fake from someone like this is MUCH less. There is NO way to 100% guarantee you avoid buying a fake, it can't be done. The best thing to do is educate yourself and buy from someone trusted.
If one follows the age-old and oft repeated adage of ONLY BUYING from a trusted dealer, verifying the slab by number lookup, studying the slab for authenticity, and not buying raw Morgans, they can minimize, and the key word is MINIMIZE the risk to the point that it is extremely remote on getting a counterfeit.