The driver leans his head against the driver’s window and sets the mirror so that the side of the vehicle is just visible. Then, the driver leans to the middle of the vehicle (between the front seats) and does the same thing with the passenger-side mirror.
It is necessary to check that the mirrors are properly set, of course. Here’s how that is done: Watch a vehicle approach in the rear-view mirror. It should appear in the side-view mirror before it leaves the rear-view mirror. And then it should appear in the driver’s peripheral vision before it leaves the side-view mirror.
Ahora le están poniendo radares a los carros para resolver este mismo problema.