In order to check the strength and seal of a fitting that has been swaged on a tubular part, the difference between the nominal inside diameter of the tube and the larger diameter of an inside circumferential groove formed in such tube by swaging of the tube wall radially outwardly into a mating groove of the fitting is measured by a handheld gauge apparatus that includes a probe adapted to be inserted into an open end of the tube adjacent the fitting to perform successive, comparison measurements of the tube's interior wall surface, first at a location axially spaced from the swaged groove, and secondly at the groove. The probe includes a cylindrically shaped probe head of a diameter less than that of the tube's nominal inside diameter so that the probe head can be inserted and cocked at a slight angle relative to the tube axis, and a pivoted arm having a feeler tip disposed in a slotted opening in the probe head and so mounted as to be movable transversely relative to the circumference of the probe head. The feeler tip is displaced to contact the interior wall surface of the tube, initially at a location axially spaced from the groove in order to zero the gauge, and thereafter at the location of the groove to measure its relative depth, while the probe is held so that at both measurement locations the probe head is maintained at the same cocked angle relative to the tube axis.