Late add …
From a couple miles altitude and a couple miles lateral offset even with an unobstructed side view a 767-300 and a 737-300/400 look almost identical. Likewise the 777-200 & 767-300. The biggest clue is how quickly the airplane goes through it’s own length. But unless you’re real confident what speed they’re going, even that isn’t real definitive. Wingletted 767s look a lot like wingletted 737s.
Contrast that with the days when substantially every airliner in the US was either a 707, 727, stubby 737, 747, or DC-8, DC-9, or DC-10. It’d be easy to confuse a 707 with a (non-stretched) DC-8 at a distance. Otherwise, each was real distinctly, obviously, grossly different.
Just one more change since the Olden Dayes.