You can check your density by going to a free tool such as
www.keyworddensity.com (lots of different free tools available).
Enter your website page URL and it should come up with a list of words found on your site and a density % for each. Ideally you're looking for your keywords to come up with a density of 4-8%.
Generally density over 8% is not seen as favourable by the search engines and they might think you're spam. This means you've used the word too many times.
One word of advice though - make sure your webpages read ok from a visitor point of view - after all there's no point cramming them with keywords and making them ultra SEO friendly but they're complete rubbish to someone visiting your site.