You can use this tool to check the density of keywords.
There is an other similar tool that I use Here instead and to me, this is one of the most important tools because of its performance depend allot the importance of your website's content in the search engines. this tool will measure the density of your web page's content and will give you a choice of keywords with the right density content, you will not need to put a lot of keywords in your meta tag, only a few of them with the right density will be enough.