Christian Langreiter offers a tool comparing results of Google.com and Google.cn. E.g. a seach for tiananmen square shows that the Google.com top result, a page on Wikipedia, doesn’t appear on Google.cn at all... whereas the Google.cn top result only appears in Google.com’s 17th position. (Indeed, a search for that Wikipedia page on Google.cn using the site operator brings up the Chinese “sorry, censored” text.)