Hold an eyepiece up to a light (other than the Sun) and bring it close to your eye. The distance between your eye and the eyepiece at the point where you first see the entire black circular field stop (allowing you to see the maximum field of view) is the eye relief.
Eye relief should be fairly long for comfortable viewing, particularly if you must wear eyeglasses, where you will need a minimum of 15mm of eye relief to seethe entire field of view. Even if you don't wear glasses, very short eye relief can allow your eyelashes to touch the eyepiece, transferring oils from your lashes to the lens. This can eventually damage eyepiece coatings, so examine your eyepieces frequently and clean them as needed.
Eye relief usually decreases as eyepiece focal lengths get shorter, so if you wear glasses you may want to use a Barlow lens in conjunction with a longer focal length eyepiece to get both satisfactory eye relief and high power. In any case,if you wear glasses, your inability to get your eye close to the eyepiece will not result in out-of-focus images, merely a narrowing of the field of view.
Manufacturers measure eye relief from the last surface of the eyepiece eye lens(the lens closest to your eye) to where the image is formed. While this is the technically accurate way to measure eye relief, it can often lead to misleadingly long eye relief figures. Since the eye lens is usually recessed within the eyepiece body to protect the lens from damage, you can't use that portion of the eye relief that's recessed within the eyepiece body. To calculate the usable eye relief, we measure the eye relief from the rim of the eyepiece or eyeguard (where an observer's eye or eyeglasses touch, which limits how close you can get to the eye lens) to where the image forms. Accordingly, our eye relief figures are usually shorter than the manufacturers' technically correct measurements, but we feel our figures are more realistic.