[Juha]

Hello Geoff
my flying experiences have been only as a passenger, so no practical experience on my ability to discern aircraft markings while flying. But
I know the results of the FiAF tests on the visibility of its markings and those of designed to give lower visibility to national markings but still be reasonably visible on cloudy weather slightly farther away than 600 m (the firing range of a fighter)

and at least in theory Naked eye: Angular resolution: about 1 arcminute, approximately 0.02° or 0.0003 radians, which corresponds to 0.3 m at a 1 km distance. That's from Wiki, so certain amount of sceptism maybe needed.

I have a copy of Fighting in the air. The official combat technique instructions for British fighter pilots, 1916-1945 and from the 1943 Bag the Hun! instructions I would say that from 500 y and 30 deg. off (above or above and side) it might well be possible to discern the over wing Balkenkreuz but the fuselage Balkenkreuz might be more difficult to discern even from a much shorter distance if on the shadow side and the angle off is small. This is of course speculation on my part.

That one side had crosses and the other roundels or stars made things easier in ETO and on the Eastern Front.

Finns shrank their markings on bombers after the Winter War so that the max dimeter became 1 m. Pilots had strongly protested that our national markings were too visible. Even so no confirmed FiAF "own goals" (there are some rumours on one case but they are not substantiated) during the 3 ½ years of wars but FiAF shot down at least two German bombers (one He 111 at night and one Ju 188).