\windows\system32\Regsvr32.exe is the tool to register/unregister the dlls.
But it doesnt seem to have a -list option to see a list of what is registered.

I would say just scan through the registry for the dll filename and if it turns
up in a registry folder called "components" then I think it is registered.
You CAN have multiple occurances of the same dll work OK side by side from
different folders, but sometimes they get tangled if the registration entry
does not contain a specific folder name, OR if two dlls with the same
NAME AND PATH but on different DRIVE letters exist.

Please note, this is largely theory, just logical thinking with a little
actual first hand experience thrown in.