If you’re looking for an easy way to focus your binocular—or to check its focus—gaze up at a star. A star is unique because it is a single point of light and when you roll your focus knob back and forth there is no doubt when the image is tack sharp.
The correct way to focus your binocular is to first look through the side that that doesn’t have a diopter focus adjustment and make that image sharp with your center/main focus wheel. (This is usually the left barrel.) Then look through the other side at the same object, in this case a star, and focus it with the diopter adjustment, which is usually on the right eyepiece. No matter where it is located on the instrument, however, it will only alter the focus of one barrel of your binocular—this is the side you want to focus second. Once you do that glance through each side of the bino in turn to make sure everything is sharp. I do this check while waiting for the sun to get up during early mornings afield.