No Idea. Probably the same way they check if a car is insured/licensed etc etc etc. I'm not against elderly people driving, but there are some elderly people who just REALLY shouldn't drive. I know there are some people who just really shouldn't drive full stop, but they COULD learn the skills they need to get better.
If you have failing eyes, ears, breathing and therefore, horribly slow reactions, then you shouldn't be on the road, unless you can drive to the fullest of your ability. When stepping into a car, you owe it to those around you to be able to control it perfectly.