The project specifies a test method for determining of smooth, non-embossed plastic films (method P). It permitts to find out irregularities of thickness to be detected across the width and - in special cases - along the length of a test piece taken from the sheeting or strip. In addition, the testing according to method P serves as an auciliary to other testings, e. g. the tensile test, or the determination of the electric properties, and it is then carried out on the specimens prescribed for these testings. The irregularities of thickness are not considered at the testing according to the second method (method F). This method permitts the determination of the thickness of embossed and rough straight films.