If you come across a monster and want to know whether you have heard of it before.
Take the LOWEST from your WIS and INT.
Apply the following modifiers based upon the creature's rarity in the MM:
Common -1
Uncommon -2
Rare -4
Very Rare -8
If you make your roll, then you have heard tales of such a creature. If not, then you haven't got a clue what it is.
If you have Monster Lore NWP, then take the HIGHEST from your WIS and INT and apply the following modifiers based upon the creature's rarity in the MM:
Common +1
Uncommon +0
Rare -2
Very Rare -6
Just make the rolls yourself and then think up a reason as to why your character would know something about the creature.