Another way to get on the inside track on potentially lucrative niche domain names or site concepts is to pay attention to forums that focus on high value niches like finance or law. Track the chatter on such specialized forums and look out for new terms that come up. If such terms keep coming up, do some follow up investigation as mentioned below.
Once you have a list of potential new niche terms, use Google Adword's media planner to find the level of attention the terms are getting. If they already get some traffic, dig into their traffic patterns. Do you see it rising. Quickly do a domain search to see if the domains for the terms aren't already taken. If they are, you're too late in the game. Find other terms.
If you don't find too many sites, that's a good sign. Reserve the domain.