This gauge for mounting a screw in a cage nut or in a nut (23), with the aim of assembling at least two parts (13, 15), comprises a shank (37) with a smooth section (39) having graduations (47) and with a threaded end (41), which are similar to those of said screw. A sleeve (49) is slidably mounted on said smooth section (39). The axial length of said sleeve (49) and the distribution of said graduations (47) are such that, when the shank (37) of said gauge is screwed as far as it will go into said cage nut or into said nut (23) and when said sleeve (49) is pressed against one (13) of said two parts (13, 15), the graduation indicated by said sleeve (49) on said smooth section (39) is representative of the distance separating the thread (43) of said cage nut or said nut (23) from the outer surface of the part (13) situated facing away from this cage nut or said nut (23).