We show the benefit of considering periodic dilution rates in the chemostat model for discriminating between a growth function which does not depend on the density of the microorganisms population (such as the Monod law) and another one which does depend (such as the Contois law). This goal is achieved thanks to the measurement of the abiotic resource only. We then present a simple procedure for a robust discrimination between the two types of kinetics using a single experiment in three phases. Finally, the shape of the best periodic excitation is discussed and the method is illustrated on numerical simulations.