There really is no easy way. When an organ is installed, you can make notes of which organ IDs correspond to which organ. Once it's uninstalled though, it's not so easy. With an organ installed, you can either look at its ID on the install/uninstall page, or go to the installation package folders and open the installation package ID file in a text editor to see the name of the organ.
You can also write down which installation package ID's you still have installed correspond to which organs, then go to the cache and manually delete any numbers that don't correspond to the numbers you still have installed. If you accidentally delete the wrong cache, don't worry though. Hauptwerk will just regenerate it again the next time you load that organ.