Receptional_Andy - 9:01 am on May 2, 2008 (gmt 0)

A quick measure of competition would be to see how many other websites contained the phrase you are interested in, e.g. compare the number of results for a search for "some competition" [google.com] with one for "not at all competitive" [google.com].

Of course, much more significant than those numbers is whether or not the words and phrases are relevant to your pages, and likely to send you the right kinds of visitors. This is as much a matter of judgement as statistics.