I use the keyword highlighter in SEOBook toolbar. You have to type the word in, but it will highlight every instance of that word on a page. It will give you a quick view if you're using your key phrases a lot.
or just use "ctrl+F" in chrome (it will highlight all matches on page and write the total).
There isn't an exact percentage (or density ) you should have, it really depends on the content quality and length, the other content pages in your site (and its quality)
if its a targeted page (page you want to improve its serp), assuming it got 600+ words I would go for 4% density of main keyword , and 4%-6% of keywords diversion.
ketword- “travel Spain”
diversions- “travel in spain” “traveling Spain” “trip to spain” etc...
you can also use tools that compares your content to competitors, I used to use SheerSEO that got a nice feature for that, but my guess is that a lot of others got similar feature.
Edited by Jill, 26 May 2013 - 10:36 AM.