In the book "The Hero of Ages" Spook removes his spike and sends a messenger off to Vin. How did he know at that point to make sure the message was written in medal rather than just a piece of paper?
All of the Final Empire's agents under Elend were using memorized messages or metal-inscribed ones.
Ruin tipped his hand to get free during the events of Well of Ascension. Ever since Sazed discovered his journals explicitly altered, and compared them to Kwaan's original metal inscription at the Inquisitor home (which specifically called out that Ruin was changing the words everywhere it could), all the good guys knew that written words could not be trusted to stay the same. Within the first few chapters as Elend is moving the army around, it's mentioned that all his scouts and messengers are following the same procedure as Spook.
Actually, here's a quote from their planning meeting in Hero of Ages Chapter 12 where Spook is doing that already:
Demoux stood. “We had a steel-inscribed message from Spook while Your Majesty was away,” he said. “The lad says that the faction controlling Urteau is made up of skaa rebels.”
And here's another thought from Vin from Chapter 21 where it's made explicit.
But, could she even trust the logbook’s words? The force she had released, the thing she called Ruin, had proven that it could change things in the world. Small things, yet important ones. Like the text of a book, which was why Elend’s officers were now instructed to send all messages via memorized words or letters etched into metal.
Because Kelsier explicitly told him to use metal.
Kelsier nodded. “Listen to me. You’re going to wake soon. I need you to remember this part, even if you forget everything else about the dream. When the end comes, get people underground. Send a message to Vin. Scratch the message in metal, for anything not set in metal cannot be trusted.
Mistborn: Secret History - Part 6: Hero - Chapter 6
The reason Kelsier told him to do this when it was already common practice was because the other common option was memorization and he needed to make sure that the message was something Ruin could get to, but not easily.