You can use SPSession's fetchScrobblingStateForService:callback:
to find out if scrobbling to a particular service is enabled.
To enable scribbling if it isn't enabled, you need to call setScrobblingUserName:password:forService:callback:
to set credentials for the service, then setScrobblingState:forService:callback:
to enable it.
Set the state to SP_SCROBBLING_STATE_USE_GLOBAL_SETTING
to keep the user's global preferences in your session (which is preferred).