A checking apparatus, for example a plug gauge, for checking diameters and/or geometrical features of mechanical parts includes a support structure (1) and checking means (20,21) coupled to the support structure, with mutually movable feelers (36) for contacting the surface of the part and transducer means for providing electrical signals. The support structure includes at least two studs (11,12) that achieve a guide along a longitudinal axis, and the checking means are coupled to the studs in a removable and adjustable way along such longitudinal axis. Several checking means can be coupled to the studs. A particular embodiment foresees three flanked studs and checking means coupled to pairs of adjacent studs. An adapting system allows the feelers to self-center on the part to be checked, for example the surface of a hole.