In the smart card quality assurance field, Software Testing is the privileged way of increasing the confidence level in the implementation correctness. Software testing involves a set of activities that includes at least test data selection, test execution and output correctness checking. When testing Java Card application programming interfaces (APIs), the tester has to deal with the classical so-called oracle problem, i.e. to find a way to evaluate the correctness of the computed output. In this paper, we report on an experience in testing methods of the Oberthur Card Systems Cosmo 32 RSA V3.4 Java Card APIs by using the Symmetric Testing paradigm. This paradigm aims at using user-defined symmetry properties of methods as test oracles. We propose an experimental environment that combines random testing and symmetry checking for on-card testing of several symmetric methods of Java Card APIs. Based on this environment, we develop a semi-empirical model (a model feeded by experimental data) to help deciding when to stop testing and to assess test quality. First experimental results are reported and the extension of the approach to non-symmetric Java Card API methods is discussed.