To limit Google or Bing searches to the English Wikipedia, include this in your search string: site:en.wikipedia.org.
You can even do a Google search of Wikipedia from Wikipedia's own search box! For example, to use Google to search for pages with "geology" in the title, type this into WP's search box: google:geology+site:en.wikipedia.org
This link does the same thing: google:geology+site:en.wikipedia.org.
Another way is to use Google's or Bing's advanced settings. (To get there from Google's or Bing's search page, click on the gear icon).