The back/front channel nature is a configuration data and therefore is stored in a registered service available through the services manager.

The services are created when requests come to the CAS server.

So you need to retrieve for a service if it's a back or front channel SLO. You can do that at service creation (argument extractor) or at logout (here).
I think it's better to do at logout not to pollute the argument extractors and as they are not necessarily needed (no logout performed).