You can use Google keyword suggestion for keyword suggestiond, which will show the traffic for each suggested term. When you have spotted a keyword with high traffic, look at the number of result it brings in google. The more results, the harder it is to rank for. You can also use some paid service like wordze.com or wordtracker.com
Theres also a tool called Good Keywords that people have been using for years. Just a simple little windows freeware app that checks the overture/yahoo/google monthly search counts for terms