Service Oriented Architectures (SOAs) are increasingly being used to support the information infrastructures of organizations. SOAs are dynamic and evolve after deployment in order to adapt to changes in the requirements and infrastructure. Consequently, traditional validation approaches based on offline testing conducted before deployment are not adequate anymore, demanding for new techniques that allow testing the SOA during its whole lifecycle. In this paper we propose a SOA testing approach based on a composite service that is able to trace SOA evolution and automatically test the various services according to specific testing policies. The paper describes the architecture of the testing service and presents a concrete implementation focused on robustness testing. Results from a case study demonstrate the effectiveness of the proposed approach in discovering and testing the robustness of SOA services.