LDN - Luminis Developer Network

Does anyone know if there is a CAS method that allows you to check for a current session without automatically going through the login process if it does not exist?

I'm trying to put together an auth agent for an external app, and wanted to use CAS to enable SSO, but I don't want to use the Luminis login page because we can't have that dependancy.  If there is no current session, I just want to konw that so I can move on to check the next auth source.  If there is a session, I'll need to pull the username out.

The perfect case is that there is a method I can call (with a service name) that will either respond with a ticket, or with some error message syaing no current session. 

If nothing exists, I'll have to roll my own servlet I suppose....