The only requirement for testing a SOAP-based Web service is a description of the exposed methods of the Web service. You can find such descriptions in Web Service Description Language (WSDL) files, which are normally generated when you browse a Web-service end point or you specify special HTTP GET parameters for retrieving such files.
In the case of an ASP.NET Web service, appending ?WSDL to the URL end point of the Web service will return the WSDL file for the Web service. Other SOAP-stack implementations may use the same approach or offer WSDL files under separate URLs.