http://en.wikipedia.org/w/index.php?title=Special:Log&user=Anybody
Actually, that won't do anything because if the account was created from another account it won't show up in the new account log for that user.
That also won't work for accounts made before the user creation log was invented. The way to tell is to look at their contributions list.
If the user page link is red or blue, they exist. It if it gray, they do not.
Compare these two: http://en.wikipedia.org/wiki/Special:Contributions/Anybodyx http://en.wikipedia.org/wiki/Special:Contributions/Anybody
Angela