An apparatus for checking the diameter of crankpins (15) of a crankshaft during the working on a grinding machine comprises a V-shaped reference device (10), a measuring device (6), and a support device (4) with a first coupling element (9) rotating with respect to a support element (5) connected to the grinding wheel slide (2), and a second coupling element (12) rotating with respect to the first one and carrying the V-shaped reference device. Guiding means with a constraining linkage (20) connected to the support device, guide the engagement of the reference device with a crankpin, during the orbital motion of the latter, binding a part of the movement along a trajectory substantially parallel to the profile of the grinding wheel (1), while not interfering with the movements of the support device in the checking condition. A control device to bring the apparatus to the checking condition comprises a thrust element (40) that cooperates with the support device.