I develop a custom webservice[SOAP] in salesforce and I want to test that webservice. Is a testclass ok for that custom webservice or should I test additional things.
The test class is good for making sure your code works when you deploy initially as well as when you deploy other changes that are seemingly unrelated.
I would also recommend using a tool like SoapUI to test to make sure the response is what you are expecting.
For both test classes and SoapUI, make sure to use assertions so you are verifying that the code does what you expect it to do.