Before the invention of the mirror and before the science of vision was discovered, how did ancient humans know that they had eyes that could sense light?
That eyes were the organs for vision would have been obvious; if you cover your eyes, you can't see, for example. The notion of "light" would probably have arisen somewhat later, perhaps as a consequence of observing shadows. Ideas about the relationship between light and vision, and the actual nature of light, developed quite gradually. The Wikipedia article on light has a section on the history, which gives a good overview: http://en.wikipedia.org/wiki/Light#Historical_theories_about_light.2C_in_chronological_order
