Press the depth-of-field preview button to stop down to the current aperture setting. The diaphragm in the lens will be set to the current aperture so you can check the depth of field (range of acceptable focus) through the viewfinder.

If it is difficult to discern the depth of field, first set the lowest f-number. Then hold down the depth-of-field preview button and look through the viewfinder while turning the <> dial to the right. As the aperture gets smaller, you can see how the depth of field increases.