This will allow you to see how many other webmasters are targeting the keyword or phrase. Secondly is the Goto suggestion tool . This is the data Goto keeps which shows how many people searched for keywords or phrases in the last month. It is a great way of seeing what surfers are searching for. Here is an example of the whole process then.
Lets say the broad subject is dating.
Some keywords would be relationships, advice, date, personals, and of course dating. I can then think of combinations such as relationship,help, cyber,dating, california,personals. Then using Alta Vista and the Goto tool I can plug in these combos and get an idea of how popular they are. Using all the data at hand and common sense and logic, a decision can be made if the domain is worth registering. Then repeat this process over and over and over again using as many subjects as you can and keep drilling down deeper until you find some good ones.