A laser beam horizontal trueness testing device for a laser beam projection apparatus having a beam leveling functionality, in particular a beam self-leveling functionality, for construction and/or interior finishing work, in particular a rotation laser or a line or point laser, wherein the laser beam horizontal trueness testing device comprises • a telescope having □ an attenuation filter, □ an objective having a magnifying effect and defining an optical axis, and □ a planar image sensor arranged in an image plane of the objective and serving for capturing an image from a laser beam incident in the objective, wherein a focus is set or settable in such a way that beams running parallel to the optical axis and incident in the objective coincide and are imaged over the entire aperture of the objective in each case at a same point on the image plane and are thus detected by the image sensor in a single, identical pixel, • an inherent inclination compensator (6, 6'), and • an evaluation unit configured for automatically carrying out □ a determination of an image position of the laser beam captured in the image on the basis of image processing and □ a quantification of the laser beam horizontal trueness with a translation of the determined image position into a laser beam inclination value on the basis of a translation rule related to calibration parameters dependent on a position of the image sensor in the telescope. • a telescope having □ an attenuation filter, □ an objective having a magnifying effect and defining an optical axis, and □ a planar image sensor arranged in an image plane of the objective and serving for capturing an image from a laser beam incident in the objective, wherein a focus is set or settable in such a way that beams running parallel to the optical axis and incident in the objective coincide and are imaged over the entire aperture of the objective in each case at a same point on the image plane and are thus detected by the image sensor in a single, identical pixel, □ an attenuation filter, □ an objective having a magnifying effect and defining an optical axis, and □ a planar image sensor arranged in an image plane of the objective and serving for capturing an image from a laser beam incident in the objective, wherein a focus is set or settable in such a way that beams running parallel to the optical axis and incident in the objective coincide and are imaged over the entire aperture of the objective in each case at a same point on the image plane and are thus detected by the image sensor in a single, identical pixel, • an inherent inclination compensator (6, 6'), and • an evaluation unit configured for automatically carrying out □ a determination of an image position of the laser beam captured in the image on the basis of image processing and □ a quantification of the laser beam horizontal trueness with a translation of the determined image position into a laser beam inclination value on the basis of a translation rule related to calibration parameters dependent on a position of the image sensor in the telescope. □ a determination of an image position of the laser beam captured in the image on the basis of image processing and □ a quantification of the laser beam horizontal trueness with a translation of the determined image position into a laser beam inclination value on the basis of a translation rule related to calibration parameters dependent on a position of the image sensor in the telescope.