DRM is always undocumented.... jeez
We use a particular variant of Adobe DRM ('Passhash') that nobody else has picked up... yet.
If you want to know which variant of Adobe DRM then:
rights->licenseToken->user will not exist with Passhash based DRM (that we use for Trade), but will with ACS4 Classic licenses (that we use for Education).
If you want to know where you got the book from then operatorURL is your best bet f, but if the back end server is in a middle man role this won't work as well. For instance a book that I get from the San Jose Public Library will have an Overdrive or NetLibrary operatorURL, and not a San Jose Public Library URL. (There are similar arrangements for purchasing).