I've got the Layers Manager open and I am unable to some layers

I get the message:

Unable to delet layer. A layer cannot be deleted if itis active, referenced by another element, orLayer 0 or Defpoints.

the layers I want to delete are not 0 or Defpoints, are not active and have nothing that can be seen or selected. 

how do I delete them?

how do I see if anything is referenced by a layer to check that (I don't think this is the case at the moment)