This is your primary troubleshooting tool. Select a user and a computer, and GPMC will generate a report showing exactly which policies applied and, more importantly, If a user calls you asking, "Why isn't my drive mapping working?", the Results report will tell you if the GPO was denied due to permissions or filtered out.
You will need to install the RSAT (Remote Server Administration Tools) . Once installed, GPMC will appear in the Administrative Tools folder. gpmc console