When Fletcher asks for a bathroom break, citing that holding it is bad for you (and in particular for a man's ability to become aroused), he is asked if this is true. His next line is, "It must be!" Simply, he knows it's true because he was able to say it on a day when he can't tell any lies.
I used to think that when Fletcher found out that his son wished he could be incapable of lying for 24 hours, they why could Fletcher unwish it. The reason Fletcher couldn't do it himself because it would only work on birthdays.