It’s really as simple as asking your GM if your character would know this. “Hey GM, would my character know if the troll is weak to fire?” and you’ll either get “No, your character is unfamiliar with this region and it creatures” or “Yes, your friend in the town guard recited his tale of falling such a beast at your last posting”. A lot of people enjoy this game to role-play, and using knowledge your character wouldn’t have can take the fun out of it.