A method for checking the diameter of a cylindrical part, such as a crankpin of a crankshaft, in orbital motion in a numerical control grinding machine, by means of a checking apparatus with a Vee-shaped reference device for cooperating with the crankpin, a support device movably carrying the Vee-shaped reference device, and a measuring device movable with the Vee-shaped reference device. The Vee-shaped reference device is brought, from a rest position, into contact with the crankpin, and the contact, that defines a checking condition, is maintained by gravity during the orbital motion of the crankpin.