How can you tell what position the flaps are in whilst on a commercial aircraft?
If you are seated behind the wings, but close, you can usually see black markings (thick line and a number) near the inboard side of the engine pylon which indicate their extension. Some might not be visible when the wing is in cruise configuration.

Same goes for leading edge slats, only you'd need to be seated near the overwing emergency exit to see those.

I've personally seen them on both Airbus and Embraer jets (no recent flights on Boeing airframes so can't confirm if they mark them too).

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