In times gone by, drivers used to be issued with a fares and tickets manual (as well as a faretable) - we've had some donated to my local enthusiast club. This contained not only details of each type of ticket accepted, but usually also sample images or descriptions of colours. This was especially handy where there was more variety in ticket machines, often heavily reliant on codes and numbers rather than today's "full text" issues, and scholars passes.
Naturally nowadays it wouldn't be a paper issue but something accessible via the company intranet, but do such things still exist? As Darloscott says in message #40, as ticketing has become more complicated, it is harder and harder for drivers to identify the tickets they should and shouldn't be accepting, especially if they don't see them very often. We are all human, after all!