A system for monitoring telephone cable insulation and jacket wall thickness and centering during manufacture consists of four capacitive probe segments spaced at 90* intervals with a dielectric gap separating adjacent segments. The probe is flanked by guard tubes which along with each segment are connected to a switch capable of placing any or all segments at guard potential. In various configurations ordered by the switch, capacitance measurements are made to yield indicia of coaxial capacitance, eccentricity and ovality.