Web services in .Net environment utilizes SAO at the client end, passes through SAO process and received at the server end again through SAO. In the client end, the data is prepared in XML format and in the receiving end also the data is recieved in form of XML.
To test where the data input, transmission and receipt are correct, understand the format in which XML framework is maintained. Also understand the business functionality of the application. For the application specialist also, he is blind about the transmission process as it is designed by Microsoft. For you it is pure black box to test the data going from the client to server through XML. Therefore database testing along with business functionality testing is done.
Yes you can use Soap UI for the testing of web services also you can ask development team to create a interface so that you can throw data in the web service.