I’ve been musing on this question a bit and have devised this simple system based on the Frequency statistic in the AD&D 1e Monster Manual.
|Frequency||% Know the Answer||% Know Something Useful About It|
Add 10% to the chance to Know the Answer for each point of Intelligence Bonus.
Add 5% to the chance to Know Something Useful for each point of Wisdom Bonus.
What knowledge is considered Common/ Uncommon/ Rare/ Very Rare will vary by campaign, social class, etc and needs to be adjudicated on the fly.
Useful information comes up when the character doesn't know the answer to the question. As in “You don't know what the creature is, but it looks dangerous” or “You don't know how the Burgomiester is getting rich; but you heard a woman in the tavern complain about wagon traffic in the middle of the night.”
Of course, roll the dice behind the screen and make up a believable tale when they fail.
The image is from the Dungeon Masters Guide (Advanced D&D) -copyright TSR Games 1979, and is used to illustrate one of the issues that occurs when playing with those and similar rules.