How do you tell the CAS server whether you want plain text (CAS 1.0) or XML (CAS 2.0) response?
A: You choose by making the request either to the '.../validate' or '.../serviceValidate' URL. The older protocol supports only the CAS 1.0 functionality, which is left around as the legacy '.../validate' URL.
Originally answered by: Shawn Bayern on the CAS mailing list.