Getting CLSID for a DLL file?

I want to create a small app to add and remove user defined context menu entries from the registry. To make this, somehow I need to get the CLSID of an arbitrary DLL so I can back up previous entries if they exist before writing new ones.

Although regsrv32 somehow manages to create this magical number, I didn't find any way to get that number myself.

I hope there is something better than this:

1. scanning registry for the DLL name
2. if not found, register it, scan again, and then unregister it again

If the DLL has been renamed, I can see a possibility for problems.