An inspection device and a method of checking on correct functioning of the inspection process of containers such as bottles, cans or the like, where a container to be inspected is detected optically at least in part, and the image data thus detected is analyzed with an analysis program to detect defective containers. An especially simple and reproducible function test of the analysis program is made possible by the fact that reference image data is input into the analysis program, this reference image data corresponding structurally to the detected image data and being analyzed by the analysis program with the same parameter settings of the analysis program.