Could you tell that someone is blind by looking at their eyes long enough?

No: In general, you would not be able to tell. Many eye conditions such as retinal detachment or macular degeneration do not have any outward abnormalities of the eye. If someone has very poor vision, there may be abnormal eye movements called nystagmus that would be apparent if you looked closely. Scarring on the outside of the eye, the cornea would also be apparent