There are various ways to call a man a liar. One way is just to scream it at him, which doesn't prove anything. Another is to establish facts by long and patient investigation. Still another way is not to call him a liar at all — let him do it himself.
On his work on Our Secret Weapon, as quoted in "Mystery Story Writer Turns Detective, Finding Axis Lies; Rex Stout, Creator of Nero Wolfe, Using Our Secret Weapon — Truth" by Trudi McCullough in The Milwaukee Journal (30 September 1942)