Yeah, sometimes it's impossible to feel just one brake assembly not working while underway. Heat measurement is a great indicator and a guy can check for magnet function with a compass or other magnetic indicator. I think the best way around this is to make sure all your wiring is good and to do a yearly inspection of the assemblies. Since most trailer brakes aren't self adjusting, adjusting should go along with the inspection.
I guess I've had great luck with shoe/drum brakes, none of mine have mechanically failed. In the old days, the hydraulic side of things was the first to fail.
