You can test the block by entering a page that doesn't exist, e.g. http://www.somewebsite.com/some-interesting-page. Of course you should append the URL with some keywords that actually exist on your website, otherwise the search block won't find any relevant pages!
Be sure you go to a page on your website that doesn't exist. Still not getting anything? Please enable the debug mode to see which keywords are being used to search for relating pages.