said 1 year, 3 months ago:
To determine which words a competitor is using on their site, you only need to look at their site!
If they’ve done some SEO, you can expect to find their targeted keywords in their page titles, headers, body copy and intrasite links.
@roryramsden is also right on his tip using the Google Keyword Tool.
You can also view source. Even though most SEO agree that meta-keywords are worthless, you still find companies “showing their hand” by putting all their targeted keywords in the meta-tags for each page.
Something to keep in mind: just because a competitor is targeting specific words doesn’t mean that they know what they’re doing, or that those are the best keywords for you!