Recall lore, history, faction politics, creature behavior, or the meaning of a dead language.
He knew which empire had built the bridge, why it had been abandoned, and roughly what had killed the garrison. He knew this because someone had bothered to write it down, and because he was the sort of person who read things other people called boring. Most knowledge has no immediate use. Some of it will save your life on a specific afternoon three years from now, and you will not know in advance which kind you have been reading.
Knowledge is the skill of recalling and applying what you have learned — history, natural philosophy, faction politics, creature behavior, foreign customs, and dead languages. Make a Knowledge (Intellect) Trial when the question is whether you know something: a noble house’s allegiances, what a creature’s tracks mean, the protocol of a court you have never visited, the rough translation of a text in a tongue that should not be legible. For actively searching out what isn’t yet known, see Discovery (Intellect); for the arcane and supernatural layer, see Mysticism (Intellect).
Examples: naming a heraldic device and its bearer’s reputation; knowing what a particular poison’s smell indicates before anyone drinks it; recognizing an extinct cult’s iconography on a freshly carved door; translating enough of a dead language to know which part of the text is a warning.