You can lookup a users’s GUID using WorkGroup Manager, but what fun is that?
Here are just a few ways you can lookup a GUID
Lookup a single user: 10.7 & 10.8 Only:
sudo /Applications/Server.app/Contents/ServerRoot/usr/bin/cvt_mail_data -i bobsmith
How handy it would be to have a list of all users and their GUID?
dscl /Search -list /Users GeneratedUID
Above returns all of the sytem users, its cleaner if we omit them:
dscl /Search -list /Users GeneratedUID | grep -v '^_'