To measure the flatness of the sole of a plane many people use a steel straightedge placed against the sole. For various reasons we don’t think this is an ideal method. The straightedge is seldom very straight, and even with the naked eye you will be able to see deviations of several thousandths of an inch along the edge, which is not good enough. Rather place the plane on a granite plate and measure around the edges at various places with a feeler gauge. With high-priced metal planes, the deviation in flatness is normally around 0.05 mm, and with mid-priced planes it might be about 0.1 mm.
For measuring the squareness between the sole and the sides, the sole of the plane should not be used as a reference, as there may be small differences in its flatness at different points on the surface. This task is best performed by placing the plane on a granite surface plate in order to find a neutral reference. With a precision metal working square, placed on the plate against the side of the plane, you will be able to spot precisely any deviation from square. The body of the square must be flat against the granite plate. With a feeler gauge, measure any deviation between the blade of the square and the side of the plane. The thinnest feeler blade of your gauge that will not fit between the blade and the side is used as the deviation. With high-priced metal planes, the deviation is normally about 0.06 mm, with mid-priced planes it is about 0.15 mm.
All manufacturers have, or should have, their own tolerances for these measurements. These can differ from the figures we have given here. If you are in doubt, it is best to contact the manufacturer directly.