Mnemonics to distinguish popular airplane models
https://aviation.stackexchange.com/a/2742/1084

The easiest thing is looking at the fin for small types! The A320 from Airbus doesn’t have a dorsal fin, the B737 from Boeing has dorsal fin and the vertical- and horizontal stabilizer reaches to the far end of the fuselage.

The big ones are generally easy because the B747, A380, A340 and MD11 are distinguished by the number of engines and the hump (B747) for the half, full (A380) or none upper deck (A340).

The A350 is similar to the B787, but it helps that A350 wears a “black mask” around the cockpit. Therefore the A350 looks like Räuber Hotzenplotz or Zoro or whatever you prefer in your part of the world.

The landing gear is barley visible and different and swapable engine types are also difficult. The nose of Airbus planes is more round, while a nose from Boeing is usually more pointy.

For me the problematic ones are the A330 which is shorter than A350 and B777 which is longer. But there are also the B757 (small body) and B767 (wide body). For this ones you need probably take a close look and it helps if a different type is next to them, to get an impression of relative sizes.

A350 - Callsign Bandit?