You can't believe your eyes: Joe DiMaggio's signature on a mint-condition baseball, elegantly etched in blue, placed perfectly between the red stitches. Asking price is $250 - firm.Is it the real thing? Or is it a counterfeit? There are some simple and obvious precautions when buying autographed sports memorabilia, say dealers and collectors. In the case of a signature, the surest way to know it is authentic is to get the autograph in person. Short of that, especially when it is an expensive purchase, buy only from large, well-known dealers. One foolproof way to determine the credibility of a dealer is to find out if he or she offers a lifetime, money-back guarantee should the authenticity of the item sold ever be questioned, says Gary Gilbert, owner of Gilbert's Sports Nostalgia store in Needham, Mass. He will not buy anything he sells unless he is 100 percent certain. For the really rare item, obtain a second opinion from an expert. Unless you are an expert yourself, be very skeptical of high-priced "bargains," warns Karen Keane, managing director of Skinner, Inc., a Massachusetts-based auction house. Sales at auctions by reputable houses have built-in safeguards against fraud, says Steve Ellingobe, publisher of "Sports Collectors Digest." In comparison to the size of the market, he adds, "counterfeiting is not a serious problem for the hobbyist."