The ability to lie is something that is human by nature and the only species in the universe that lies is without a doubt the human species. The order have a very simple way of knowing when humans lie. They simply check for certain patterns of behavior that are not even known by humans. Even the greatest experts in the art of detecting liars are not able to use these methods because they are too advanced for the human brain to use.
There are many people in the world who have tried to lie to members of the order, but even the best human liar on this planet is never going to be able to fool a member of the order. If there is one thing that the order can expect, is that humans are never going to stop being liars, unless their nature stops being selfish, which is currently not the case.