[tjr]

Some recommend using the Google keyword planner tool to ascertain if very many people seem to be searching for whatever it is that you have in mind.

Obviously this works better for some categories of ideas than others; if your idea is innovative and unique, the fact that nobody is searching for it doesn't necessarily mean anything. On the other hand, few people searching for "royalty-free polka music" is probably a good indicator of a not-very-valuable idea...