In Windows Vista/Server 2008 and newer operation systems from Microsoft the userenv.log file which was logging Group Policy processing information in Windows 2000/XP are replaced by a new event log named Group Policy. You can find it in the Event Viewer when you browse to Applications and Services Logs/Microsoft/Windows/GroupPolicy.
The event categories found in the Group Policy event log:
This really makes Group Policy troubleshooting much easier!
In addition to checking out the Group Policy event log on the client, I would also recommend the use of the Group Policy Modeling (simulating what is supposed to happen) and Group Policy Results (connecting to the client to see what did happen) wizards when troubleshooting Group Policy: