A recent question about a dogs eye, got me wondering. How does a veterinary specialist perform a vision test on a dog? When I go to the eye doctor, the majority of the testing is based on my verbal responses.
Obviously, you can't get a dog to read an eye chart, but there are other means of determining visual acuity. In particular, retinoscopy can be used to detect the reaction in the dogs eye. There are also some more manual tests, one that you can do at home, using room layouts and light and dark rooms. If the dog navigates okay in both conditions, then eyesight is probably okay, but if he struggles in the dark then a vision issue is likely. Obviously, if he struggles when it's light, then he has a vision issue for sure.
Bear in mind that a dog's vision is actually less than a humans and they rely on other senses such as smell and hearing to help move around, so loss of vision is a problem, but potentially less for them than us.