Try to deal only with reliable dealers. One indicator of a reliable dealer is the length of time he has been in business. This certainly does not mean that new businesses are not reliable but the longer one has been in business without problems the more likely he is to have a good reputation. Be wary of dealers who have the most, the best and the cheapest. Very rarely does an honest dealer promote their business on such claims.
Quality is much more important then quantity when shopping for art.
Request a written guarantee of authenticity on any art which you buy. If the dealer refuses, don't buy from him. Always request and keep the receipt of purchase.