In the end, it all comes down to how many searches are done for a certain product or service. Yahoo Sponsored Search has developed a good tool where you can enter a search term (e.g. Carpet Cleaner.) to see how often it is searched on Yahoo. Since Yahoo represents about one-third of all searches, you should multiply the number by 3 to get a total number. Remember, though, this number represents total searches nationwide, but it is a good indicator of the popularity of a category.