On M3, each bright frame changes automatically depenging on the lens on the camera. And on the later model, there are notches in the viewfinder to see depth of field.
When double images are inside the width of notches, the subject is in focus.
The width of the upper notch shows depth of field when the aperture is f16, and that of the bottom one shows depth of field when the aperture is f 5.6.