What document you mean? About c++ classes, NMEA2000 library or NMEA2000 protocol? For NMEA2000 library there the one inside documents, but there is not much about tDeviceList. For NMEA2000 protocol I have not found good free document. Information is around in small pieces.
But using tDeviceList is far more easier than try to do it yourself from the scratch. And about your original question "if one of two similar devices is alive on the bus", what you actually mean with that? How do you define "similar device"? Do you know their product code? Or do you know the PGN, they should send? For both questions tDeviceList is an simple solution, because it collects all device information automatically just by joining it to NMEA2000 object as I wrote earlier and as it is on DeviceAnalyzer.
I prefer you to study how to use classes. You do not need to write them, but you need to use them.