It depends on the nature and gravity of your imperfect eyes. All eye defects are not identical. In case of serious faults, you will never know if the camera is clean.
With slight defects that produce a slight blur, you can focus by observing that the image improves by turning the focus ring. To the point where it starts to get worse, reverse the direction in which you turn the ring. The image will improve again and then worsen. Back off slightly and take the picture. If in doubt, you can use the media. If you continue to miss the focus, your aperture may be too wide, stop a little to increase the depth of field.
As indicated by mattdm, you focus on the frosted glass, and not through the optical system of the lens, so you can not compensate for blurred images in the viewfinder caused by a blur. The frosted glass in digital SLRs is difficult to focus, even with good vision. Consider switching to a system with maximum focus or other debugging aid.
The eye has muscles to concentrate the lens that are partially controlled by the consciousness. In addition, strabismus deforms the eye (which can improve focus) and narrows the aperture (improves depth of field). These compensatory actions are tiring. To have a clear vision is not. Consider consulting an optometrist or ophthalmologist (who understands physiology and vision) instead of an optician (who understands optics).