What does this black triangle placard on board an A320 denote?
If there is a problem on-board and the crew is unsure about the condition of the aircraft, the triangle indicates what window should be used to inspect flaps, slats or engines or look out for icing.

This saves important time in case of an emergency, as the crew doesn't has to search the right window to look out.

For example if there is a failure of the flaps condition display, it has to be verified visually that the flaps are extended properly. From the window with the triangle, this can be seen:



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Now the crew can compare the position of the flaps switch in the cockpit, with the actual extension of the flaps.



There are usually four of these triangles in the cabin. One to inspect the forward of the wing for icing, the slats and the engine on each side and one at the back of the wing for the flaps.

I have searched for cabin pictures and found this 360° cabin view by SAS.

The following picture shows the triangle on an Airbus A330 at the front of the wing. You can see a part of the engine through the window.



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And this picture shows the rear triangle.



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