I distinguish between two kinds of meta-game knowlege. That which the Character *could* know, and that which he absolutely could *not* know.

Things the Character could know are anything having to do with common world knowledge, or things which could be figured out with intuition, observation or common sense, such as how to bash a skeleton.

Things that could *not* be known by a specific character would include secrets, such as Who Killed Larraby in the Castle of Rain, or things that no one could intuit, such as the weak spot on the Smaug's belly, whether or not there's another continent that's been undiscovered, or why the King of the Dwarves decided to smelt down the Axe of Gorund, which only he knows. That sort of thing.

If there is ever a meta-game challenge by the players then I would resort to this distinction, and if it's something the Character *could* know then I usually let it slide. However, if it really makes some sort of difference to a particular Player (or to the story in my opinion as GM) then I could roll for it using the Character's Intelligence and the level of difficulty I think the piece of knowledge would have. So for example, using silver and/or blunt weapons against Skeletons, I rate that as pretty general world knowledge - I'd give it a 2 Difficulty as for the chance that an average intelligence Character would know it. I would use his Wisdom Bonus as the modifier for his Skill Level at knowing this. Lets say the Character in question is 3rd Level has a low Wisdom at 2 which is -1. So in this case if the challenge does come up (a Player protests that Character X wouldn't know it) then I'd roll against the General Resolution Matrix with a Total Skill Level at Knowing This of 2. The Difficulty Level is 2. So in this case he has a 50/50 chance or needs to roll a 4 or better on 1d6. If he fails then I would tell him "Your Character doesn't actually know to use Silver Weapons against undead."

In either case if the question comes up - "How did he know that?" the answer is "He heard it in a tavern from a drunk outlaw", or something along those lines.

If it's something that the Character could not know, then of course, I'd automatically overrule the player and say "Your Character doesn't know that".