In the last 10 years the atomic force microscope (AFM) has routinely been applied to the direct measurement of surface forces. A particle is often attached to the tip of the force-sensing cantilever in order to control the chemistry and the geometry of the interaction. Usually a spherical particle (colloid probe) is employed, and the measured interaction force is normalized by the radius, enabling comparisons to be made with both theory and other techniques. Here we report a simple method for characterizing the radius, surface roughness, and regularity of colloid probe particles. The colloid probe is "reverse imaged" by employing a surface with very sharp features. A standard calibration grating is suitable. No further equipment is required. The method is straightforward and will be of value to researchers employing the AFM for force measurement.