Do a Google search on one of your products. When you get the results page, look for the blue "cache" link under the listing on the Google page for your competitor. Google will show you which keywords on that page were used to draw Google's attention (they will be color coded).
Now, with that in mind, keywords are just part of the search engine puzzle and the old days of putting lots and lots of keywords in the keyword meta tag are pretty much over.
Also, keep in mind a high percentage of searches are actually multiple word searches, so "french fried lima beans" gets a better result that just "beans." Think of the phrases your customers would use to find you. You may want to visit www.wordtracker.com
and use that service, which helps develop a keyword scheme for you and it's easy to use.
There are plenty of great comments in the search engine section of this forum to get you pointed in the right direction.