Most established companies already know what words prospects use to find their product or services on the web. That is, when they have not yet heard of you. (When they know who you are, they can search for your company name, but Search Engine Optimization is about connecting with people who've never heard of you).

View the source of your successful competitor's home page and look at their keywords.

If you are not so sure what your keywords should be, you do probably know who your competitors are. Look at the home page of a competitor that appears on the first page of Google search results, and view their HTML source. (View/source or View/Page source on the browser menu). Search for the word "keywords" and look at what follows.