Which field mark is described as perhaps the most reliable identifier visible on both light and dark morphs?

Answer

Dark patch on the underside of the wing near the "wrist" joint

The dark patch on the underside of the leading edge of the wing, near the wrist joint, is perhaps the most reliable field mark visible on both light and dark individuals.

Which field mark is described as perhaps the most reliable identifier visible on both light and dark morphs?
raritybirdhawk