The same way any civilisation 'knows' anything - speculatively, one guy stares for too long, gets permanent blotches in his vision, tells his friends not to do the same... word gets around, and as far as basic reasoning capacity goes, we're no different from Neolithic hunter-gatherers, even if they might have been more inclined to rationalise it as "the sun-god cursed my eyes for gazing too long at his radiance" than "the intense light scarred my retinas".
