Configuring a composite Web service by setting endpoints reduces the cost of development, but raises the probability of a request message triggering runtime execution failures. Previous works on validation of composite Web services are not useful because the application developer cannot modify atomic/composite services and the specifications needed for validation are not always available. Therefore, in this paper, we address two issues: predicting the executability of composite Web services for each request message, and acquiring input specifications to improve the prediction. To resolve these issues, we model atomic/composite services in a formal specification. Moreover, we apply constraint acquisition algorithm to acquire input specifications of atomic Web services. We conduct an experiment in which the proposed method is applied to a composite Web service in practical use. The result shows that our method can detect almost all messages that will trigger execution failure at a rather early stage of specification acquisition.