Thanks Jeremy.
I'll take a look at casting the derived classes. That may be my best bet. Everything else gives out data indicating a connection to Revit Server or not. It is the "or not" I am having trouble with. If it is not a Revit Server central file then that still leaves you 2 choices: file-based or Collaboration for Revit. I could also play with getting the central file path and examine it. There may be string patterns that indicate which of the 3 types a central file could be.
IsModelInCloud is not an extension method of my own. I found it via intellisense in VS referencing the dlls from the original build of Revit 2016. I suppose a subsequent update nixed it.