What’s the difference between a three-star hotel and a five-star one? It depends on whom you ask. Most experts agree that the most reliable rating resources are those that have been at it the longest. Mobil Travel Guide has been rating U.S. hotels since 1958, and AAA has been publishing its rankings since 1977. Both use teams of independent inspectors to evaluate thousands of properties annually. And neither is quick to give out its highest award: Less than 2 percent of properties in the 2005 Mobil Travel Guide have five stars, and less than 1 percent of those in the AAA guide garner five diamonds. One good way to read a rating is to see if it seems in sync with the room rate. “If a property listed as a Mobil 3-star were charging $550 a night, that would raise a red flag,” says Shane O’Flaherty, vice president of quality assurance for Mobil Travel Guide.