A conical indentor is brought into contact with a test specimen cut from a rubber sole. A known force is then applied to the indentor and the depth of indentation after a specified period is recorded. A series of readings at specified locations is taken and a graph of indentation depth against distance from the bonding surface is plotted and extrapolated to the inner and outer surfaces of the sole. An undercure index for the inner surface of the sole is then calculated together with the thickness of rubber not cured satisfactorily.