This paper explains a part of a method to design assembly systems: the way to obtain a Petri net, assembly system modeling, in order to determine important features of the system (cycle time, working time of the station, ...). The method must include the multiproducts aspect and the quality aspect. This second point can be done with a rational integration of a testing strategy in the method. So, the assembly systems we modelled have testing operations. We show some examples of multiproducts assembly operations and the different types of testing operations. Then, we present their coloured temporized Petri net modeling. In the case of the testing operations, the Petri nets are stochastic. We end this paper with an example of simulation.>