Here's a crazyan idea: create a mock application that has the exact same layout as the live system and have them use it for a while.
You can then use the testing environment to see how they work under stressful conditions, or when things don't go correctly, or when multiple people are asking for help.
This should be a version of the training system. It doesn't have to include all the scenarios, but it could test how quickly they learn and adapt. You will have to be able to tell who is likely to climb the learning curve, and who will be overwhelmed by the entire process.