Focus on it at the center of the field, and move the binocular to place it at the edge. It will flare out into a V if the binocular is comatic, and will become fuzzy if the field is curved, although this distinction is academic to most users. Astigmatism is also difficult to distinguish during the day, for an astigmatic image is blurred at the edge, similar to the effect of coma and field curvature. However, a star or other point source is imaged as a short line, vertical on one side of focus, and horizontal as you rack through sharpest focus to out of focus on the other side. Astigmatism can sometimes be seen by viewing fine, perpendicular lines at the edge of the field, where one line will be sharp and the other fuzzy.
Distortion causes straight lines to appear curved at the edge of the field.
It can be seen when the edge of a building is viewed at the top and bottom of the field, or a pole is viewed at the right and left edges of the field. The lines seem to curve or flex as the image is moved across the field. Barrel distortion caused the lines to curve outward, and pincushion distortion causes them to curve inward. This is a relatively acceptable aberration that can cause discomfort during daytime use when the binoculars are used to sweep across landscapes that contain lines. Color error can consist of longitudinal color, where the lens is acting like a prism and focusing the blue rays closer than the red.