Your best protection against buying fakes is to buy from knowledgeable, reputable coin dealers. Don't buy from eBay auctions that don't allow a return period (if sellers don't guarantee their coins, you should really wonder why!?) Don't buy from sellers who hide their feedback; anybody who has good feedback has nothing to hide!
What if you follow all of the common-sense rules and still end up with a fishy-looking silver coin? The article linked below contains tips on how to detect fake silver coins. The top part of the article has some general introductory information. The middle part is a step-by-step procedure to analyze your own silver coins, and the final part is an example of applying the steps to a suspicious Silver Eagle. Silver-Coins.org.