ed,
It's not so much a "trained eye" as it is familiarity. I'm sure that you don't recognize another nearby aircraft by it's individual features. You automatically assess the overall picture and say that's a 737 or DC-9 or Cherokee or 172, etc.
Your Camel/SE5 photo was a difficult one, but if it was viewed on a video or in real life, the identification would be immediately obvious as the aircraft rolled upright.
Simon