Testing Service Oriented Architecture applications is a challenging task due to the high dynamism, the low coupling and the low testability of services. Web services, a popular implementation of services, are usually provided as black box and using testing techniques based on implementation is limited. This paper presents an approach to support the use of the structural testing technique on web service testing. The approach improves web service testability by developing web services with built-in structural testing capabilities. Testers can run test cases against such web services and obtain a coverage analysis on structural testing criteria. A set of metadata provided with the testable web service helps testers to evaluate the coverage reached and the quality of their test cases. An implementation of the approach is presented using a service called JaBUTiWS that performs instrumentation and coverage analysis of Java web services. We also present a usage scenario of the approach.