Clients need to determine if it has administrative rights so it can take corrective action (e.g. request the user enter different credentials) before it tries and fails to execute a privileged operation.
There may already be a solution, but it is not obvious.

One solution is to use the authentication token returned from CreateSession to determine the roles assigned to it, perhaps by invoking a method in the Server that returns the list of roles for that token.

Use case: a client used for configuring servers needs elevated privileges. If a user enters the wrong credentials the system should detect this and inform the user before it tries and fails to correctly configure the server.
This is important when some operations may succeed even with reduced privileges but others fail, resulting in an incorrectly configured (and possibly broken) server.