If you get stuck or a query runs forever, the operators are usually happy to help you. To find out who could be there to help you, use – the relational registry. If you have the IVORN of the service, use
SELECT role_name, email, base_role FROM rr.res_role WHERE ivoid='ivo://org.gavo.dc/__system__/tap/run'
– if all you have is the access URL, do a natural join with interfaces.
Left to the reader as an exercise