A simple way is to count number of times it has been used in an article and then divide it by total number of words of the articles. For example if keyword A is used 5 times in a 500 word article, then its density would be (5/500) * 100 = 1 %, which meas that is has been used once in every 100 words.
Yeap , very useful tool for checking the keywords density. If you are using word press there are some free plugins available, like YOAST Seo.
Keyword density is more important to know because you don't want it to be too high. But experiments have shown that increasing density has no effect on rankings, and you shouldn't worry if your density is very low as long as the targeted keyphrase is in important places.
With Wordpress there are some good plugins, you can also find some free tool online that do it for your webpages totally free, try a research on Google.
There are some tools which are helpful for finding keyword density of a web page, try using those tool, just type in Google "keyword density checker" and you would find some tools.
+1 for SeoQuake. I use it for our clients. @Op you should use this addon for firefox and you'll be amaze to the see the different aspects about your site.
http://www.slideshare.net/LinkAssistant/seo-basics-part-3-landing-panges-and-content-optimization there's a formula accodring to which you can calculate keyword density there. but i think it's overrated, just remember to use your keywords and use as many of them as would look natural in all the vital elements of the page.