they actually know this by how theirs work, and positioned on their head. Domesticated cats for instance are actually nocturnal and see in the dark very well. little light is needed, its why they are great hunters. Its also "thought" they see shades of grey, this is not true. you can equate it to being colorblind in a person. Your dog depending on breed is probably the same way. Their irises just open fucking bigger than a persons that will allow more light in, plus there other things