After you've identified several search engines that may meet your needs, you'll need to put them to the test. Take the time to download, install, and test several. (But remember to do this internally rather than on your actual Web site.) Your tests should ensure that the search engine is easy to use from a user's perspective.
Evaluate the ease of installation and maintenance of each potential package.
Find out how well they index your site and whether you run into any incompatibilities or problems.
Search using keywords that represent your typical customer inquiries, and see what results the search engine produces. Does the search produce relevant information?
Search for key products or product features and find out what pages are served. Ask various people to help you test the search engine, since different people will search in different ways. Note whether the most relevant items come up with the keywords you use. Determine whether problems reveal shortcomings of the search engine or whether they reflect oversights in how you categorize the content on your site.