The cockpit windows are a giveaway. On the pilots side windows, Airbus cuts away the top aft corner, Boeing the bottom aft corner.
Another giveaway are the wing end plates on A320 series planes.

Or you can cheat a bit. Know which airlines fly which jets.
True, some fly both makes, but few fly the same basic size of jets from both makers…i.e. 737 and A320 or 767 and A330.