Hook it up and go for a little drive and see if they work? I can't imagine jacking up the trailer to test brakes pre-trip.
Agree diagnosing problems can be a bit more difficult and to date, I've had far more "service trailer brake messages" from a plethora of IBC equipped trucks in the last 10 years than ALL other add on brake controllers for the last 30 years. I've literally dealt with 100s if not 1000s of light duty trucks in some capacity from driver to mechanic and I don't look forward to when these newer trucks get old....I'll have to buy a nother new one, lol!