it's been around 5 years since I worked on web sites but back then there was a great "free" tool to check for keyword saturation. It was very simple and even told you how many keywords - phrases you needed to include or take out to be seo. Anyone run across anything like that recently? All I'm finding are strange meters that mean nothing to me.
Keyword Research is the very important part of optimizing. Before you can do on page optimization you need to find the right keyword first. Not all of the keywords that is ranking for your site are useful and will bring lot of targeted traffic. That is why you need to do keyword research by using keywords suggestion tool. Google suggestion tool is the best tool to find related and synonyms of keywords. After getting related keywords, find the traffic of each keywords and the number of search result in search engine then compute for KEI of each keywords. The formula of KEI is [ KEI = (traffic/search result)*traffic ] . It is better to target keywords that has high KEI which is greater than 1. If the KEI is less than 1 it only means that the keywords is very competitive or useless to target. Keyword Suggestion tools that you can use are: http://freekeywords.wordtracker.com/ http://keyworddiscovery.com/keywords.html http://tools.seobook.com/keyword-tools/seobook/
Be carefull when you are choosing your targeted keywords. Among those that mentionted before, have a look at these http://www.backlink.gr/tools/cat/keyword-tools/
To estimate keyword saturation, you have to use proper SEO tools. Almost all of them are not free, some have free versions but can't provide expected service for you. You can give a search and see the result, but the free softwares are not worth it. There are lots of SEO tools that can really help you, many of them are low priced, and some of them offer free trials. That means you don't have to pay if you are not impressed with their service. You can try this free trial for ur site.