[MrB]

Special equipment is not really needed, except that it's best to mount the camera on a tripod. Lay a 30cm ruler (one with mm markings) on the table in line with the axis of the lens, then tilt the camera and lens down at roughly 45 degrees, and at about 50cm away from the ruler. Set the lens to a wide aperture, and adjust the position of the ruler so that the 15 cm mark is in the centre of the field of view. Set the camera to use the centre focus point, and use the camera's self-timer to eliminate camera shake when taking the shot. In the image, the "15" and its line mark should be sharp, with blur increasing towards 14 and 16. If not, adjust the focus calibration in the camera, and repeat as necessary.

Cheers.
Philip