If you own a website, set up Google webmaster tools for it, and you will see which keywords you rank for.
If you don't own a site, and want to find out which keywords a website ranks for, use this free online tool for a fast check:
http://www.webdatah.com/
I think it only returns up to 10 keywords, even when a site is listed for more keywords, but it's excellent for a fast check of any website, especially if you don't have access to it, e.g your competitor's.
There are many other rank checking tools, but for 99.9% you have to type in a keyword you want to check - and you never know whether a site might rank for other keywords you missed to think of.
I like the above tool, because it's the only one I ever found that checks for a website in general and finds up to any 10 keywords a website is ranking for.
A second good free online tool is
http://www.keywordspy.com. But the free version is limited.
So using these two free tools will help you answer your question.
BTW: I don't own these websites and I'm not affiliated with them.