A method for production control in stretch blow molding of plastic containers in a blow molding machine, and a test preform for the use therewith, and a blow molding machine for performing the method. At least one test preform with a regular and/or in particular equidistant, printed test pattern is fed into a stream of product preforms entering the blowing molding machine. Furthermore, the product preforms and the test preform are in the continuous operation of the blow molding machine blown to form product containers and to form a test container and the test container is imaged with at least one camera. The product quality during the ongoing test operation can thereby with imaged test container be performed flexibly and selectively.