If a person is legally blind, he has central visual acuity of 20/200 or less in the better eye with best correction. In other words, it indicates that with the use of a correcting lens, the legally blind individual have to see an object clearly within 20 feet away from it while a normally sighted person could see the object clearly from 200 feet. I do not think it right to prove a person is legally blind because it is not polite.
The exact definition of legally blindness is that the best level of the vision that you can achieve with the vision correction is just 20 degree. Many people with short sighted symptom feel that they could just see a foot in front of their face when they take off the glasses or contact lenses with prescription. However, those people even wearing the eyeglasses with the exact prescription will only see the 20 degree. That is to say their eyes can't get the full exact vision before them. If they accept the eye exams, they will be diagnosed whether they have the legally blindness or not.
Well, there are more and more people who have suffered the legal blindness. generally speaking, legal blindness means a person has a vision that is 20/200 in a person's best eye with correction. But on the other hand, you should know that when you have the legal blindness, the visual field is limited to less than 20 degrees. And legal blindness does not mean that a person cannot see at all. People who are legally blind often have some vision, but their field of vision may be very narrow or blurry. Or they may have blind spots that glasses cannot correct.