For me its one of looking at the clues. Take the challenge above, the throttles and engine instruments shout trimotor. Its a stressed metal skin construction with a smooth skin so it isn't a Junkers, Ford or Fokker (just watch it turn out to be one of these
) which will narrow the search down a bit. Looking for a fuselage shape that fits what you can see will be part of it but for this one I feel that the main point of identification will be the windows, that asymetrical DV panel, the step between the windscreen and the side panels and is that a kidney shaped window in the roof?
Ultimately it is likely to come down to how big your archive resource is (books, magazines, photo's etc). The internet is not that great on the really exotic subjects but challenges like this will expand its knowledge.