After you have installed the Disk Deduplication functionality into Windows Server 2012, you also get a new program called DDPEval.exe By running this utility, you can get an estimate of you disk capacity savings should you implement disk deduplication on a volume. In the example below, you can see how much space would be saved by implementing disk deduplication on this volume.
This volume would greatly benefit from disk deduplication. This command will only run on a volume that you have not enabled disk deduplication. If you run it after you have enabled it, you get the following error: