Is your question how to tell if the unit is a Napco unit? Or is your question how to tell a factory Napco truck from a dealer installed Napco truck?
Here is some info I got from
www.napco4x4.org before buying my Napco:
If it was a factory Napco, then they didn't put the Napco badges on the fenders. The transfer case would be riveted to the frame cross member, not bolted. And the GVW on the information plaque on the driver's side door would be higher than for a 2WD.
If it is a Napco, the tie rod will be behind the axle, not in front like on most of the later model 4WD stuff. And the word Napco will be cast into the axle, but if it's dirty you may not see it. The steering box will be in it's stock location inside the frame, not outside like some of the add on jobs I've seen. The transfer case will be a Spicer or Napco model 23, and should have a red information tag on it.