Actually, Google Webmaster Tools (GWT) is good for both. Its main role is to tell you your ranking for the search terms that people used to find your site. But, less obviously, it also tells you what search terms were used by the people who did not find your site - the near missses, if you will. Both items of information are very valuable when it comes to optimising your content.
But I agree with the above quote. Vie, you need to tell us what you mean by "check the keywords that I am optimizing".