We have machines that can measure how well an eye sees. You might have used one if you've been to an opthalmologist, you know that big one you rest your chin on and look into a hole and you can see an image inside that may shift as the doctor makes adjustments. Last one I used had a picture of a hot air balloon in a field. However what that machine does is send light into your eye and see how it's received and sent back at the machine. We know what a good eye is supposed to be like, how the lens is supposed to focus light on a specific spot inside the eye and how our iris is supposed to move to compensate for different light levels. If your eye ball has the wrong shape, or your eye lens has the wrong shape, or your iris is not functioning properly, the light is not hitting the right spot inside your eye and your vision is impaired.

Given that this is a completely non invasive procedure, and it doesn't really require feedback from the patient despite the fact that opthalmologists may ask questions anyways, it's possible to use it on infants as well, with probably the hardest part being holding their attention long enough to make proper measurements.