In this kingdom the king really loves freedom of speech, thus he decided that dissonesty is a capital crime and sin.
When saying something, everyone involved must be cited by name and with date and exact words they spoke, sometimes even body language and tone must be cited.
Then ofcourse the listener can demand proof.
If the teller can't provide proof of what they are saying, then they have to pay a fine both to the listener and to the king.
The fine is 5% of their monthly production or 1200 hours of forced labour for the king and then for the listener, 2400 in total.
Suddenly starting a religion isn't so fun anymore and can make you into slave for life depending on how many people were listening to your tale of a talking bush on fire.
Given pre industrial technology, what are the more reliable ways to gather proof and to enforce the dissonesty law?