Ask the pay-per-clicks

Pay-per-click sites actually let you see how popular a
keyword is. They’re not being kind; they’re trying to make
money. The more webmasters bid on those keywords, the higher
the bids are going to rise—and the more money the
pay-per-clicks are going to make. FindWhat, for example, has
a Keyword Center, and Overture a Keyword Suggestion Tool.
Both are very handy, but they also require you to open an
account. That can cost a few bucks, but when you have a lot
of sites covering a lot of different areas, it’s usually
worth the expense.