Method for inspecting a weld seam between two plastic parts (11, 12) of a workpiece that are made of weldable material (18, 19), wherein the weld seam is produced by a laser radiation, wherein one of the plastic parts of the workpiece is essentially transparent to the laser radiation, whereas the other plastic part absorbs it, and wherein the quality of the weld seam produced in this way is determined by optical means, characterized in - that an additional electromagnetic inspection radiation is used that is independent of the laser radiation; - that one of the two plastic parts (11, 12) of the workpiece is also transparent to the inspection radiation; - that the inspection radiation is introduced into the interior of this transparent plastic part and hits the already solidified weld seam, wherein the beam direction of the inspection radiation onto the workpiece is coaxial to the direction of incidence of the laser beam, and wherein the inspection radiation is reflected (32; 52) both at the contact surfaces between the plastic parts (11, 12) and at the interface of the weld seam and partly exits (33, 33'; 53) the workpiece; - that the resulting exit radiation is measured; - that, during said measurement, only the exit radiation reflected by an already solidified point of the produced weld seam is detected, and the measuring process is transmitted to an evaluation unit; - that the inspection radiation (33'; 53) exiting the workpiece travels to the measuring point (36; 55) along a segment (29, 28) of the same optical path (23, 24, 35, 44, 46) that the laser light follows in the opposite direction to the weld (15; 47) in the workpiece; and - that the disturbance of the measured exit radiation (33'; 53) caused by a missing weld is detected by the evaluation unit, which triggers reactions so that the measuring result is displayed on a display device. - that an additional electromagnetic inspection radiation is used that is independent of the laser radiation; - that one of the two plastic parts (11, 12) of the workpiece is also transparent to the inspection radiation; - that the inspection radiation is introduced into the interior of this transparent plastic part and hits the already solidified weld seam, wherein the beam direction of the inspection radiation onto the workpiece is coaxial to the direction of incidence of the laser beam, and wherein the inspection radiation is reflected (32; 52) both at the contact surfaces between the plastic parts (11, 12) and at the interface of the weld seam and partly exits (33, 33'; 53) the workpiece; - that the resulting exit radiation is measured; - that, during said measurement, only the exit radiation reflected by an already solidified point of the produced weld seam is detected, and the measuring process is transmitted to an evaluation unit; - that the inspection radiation (33'; 53) exiting the workpiece travels to the measuring point (36; 55) along a segment (29, 28) of the same optical path (23, 24, 35, 44, 46) that the laser light follows in the opposite direction to the weld (15; 47) in the workpiece; and - that the disturbance of the measured exit radiation (33'; 53) caused by a missing weld is detected by the evaluation unit, which triggers reactions so that the measuring result is displayed on a display device.