The MMC snap-in rsop.msc (Resultant Set of Policies) evaluates all GPOs applied and will display them in a tree view similar to the Group Policy Editor. It is very useful if you want to know which group policy is affecting a certain setting.
When started, it will automatically create a report for the local computer with the currently logged on user. You can change the scope of the report by right clicking the root element in the tree view (Normally labeled "<user> on <machine> - RSoP") and selecting "change query". This opens a wizard allowing to select the computer (or not to list computer configuration elements) and a user to query. Selectable users are domain accounts, which have been logged on the machine previously.
After the report has been created, this tool will show a view similar to the Group Policy Management Editor, but without the sections administrative templates or preferences.
For each policy the report shows if the policy has been set (Displays the defined value or "Not defined") and the GPO causing the setting.
If a setting is defined in multiple GPOs and the source GPO is not the intended one, then the desired GPO is either not applicable to the computer/user or is overwritten by the GPO shown.