If thy eyes do not move together, the child may get a double overlapping image.
Put a colored tack in the eraser of a pencil and ask the child to follow it with his/her eyes as you move the pencil in a horizontal or vertical line across his field of vision.
The child must follow the movement of the tack with the eyes, of course, and not by turning his head. If the eyes fail to follow or follow with intermittent breaks, a tracking problem may be indicated.
Another thing to note in this activity, is whether or not the eyes move together. If they do not coordinate, the visual field may actually be splitting into two overlapping images.
This could wreak havoc when a child is attempting to learn to read. Each word may split into two overlapping words every time the child's eyes move. The child would not necessarily complain because he might assume everyone sees that way.