My wife told me the story recently of a work colleague who found a way to catch her kids in a lie 10 times out of 10. It seems this lady once watched Pinochio with all three kids and when asked "Will that happen to anyone if they lie?", answered, "Yes."
"Furthermore," she went on, "The only way to stop your nose from growing when you lie is to hold the end of it with your fingers."
From then on, each time she would ask something like "Who broke the vase?", the culprit would always be apparent. They'd be the one with their hand over their nose.
What have you tried that worked like that?