According to Deuteronomy 18:22, how could Israel know a prophet's message was not from the LORD?

Answer: It did not come true

If a prophet speaks in the LORD's name and the message does not come to pass, the LORD has not spoken it; he spoke presumptuously.

“When a prophet speaks in the name of the LORD and the message does not come to pass or come true, that is a message the LORD has not spoken. The prophet has spoken presumptuously. Do not be afraid of him.”— Deuteronomy 18:22 (BSB)