Is there a way to find out what layers are included in a reference set?
Is it possible to write a journal to list all reference sets and the layers included in each?
Reference sets don't directly reference layers; ref sets are composed of objects each of which may be on a different layer. However, when adding a component to an assembly, the user can choose "specified layer"; in which case all the objects in the ref set will end up on the specified layer in the assembly file.
If you press the information button in the reference set dialog, it will list the object types that are included in the ref set. A journal could take it a step farther and list the layer each object is on; but, it may not be useful info depending on the layer setting used in the assembly.
There is a journal here that will show all the members of the model reference set. It checks the layer of each object, it would be fairly easy to write the layers out to the info window in the same loop - if you'd like to do some tinkering.