Official answer from the official site:
1. Your two Googlefactors [the two words you use] must exist in this dictionary [
http://www.dictionary.com].
It's so easy to confirm: Google does the work! (And, Google has the final word!) In the blue bar atop your Google results, accepted terms are linked to dictionary.com, and so appear 'underlined.' No line, no link = Googlejack! (As in, You've got jack! :-)
2. Google also is the arbiter of a whack's uniqueness. Look to the right end of the blue bar atop your Google results. If you see "Results 1 - 1 of (any number),' you found exactly one hit = Googlewhack!
3. Google shows you an excerpt of the page you whacked. Look at that text. If it's merely a list of words (such as a bibliography, concordance, encyclopedia, glossary, thesaurus, or dictionary), No Whack For You!