Identify Fiend or Foe
One of the things players will inevitably ask about the screaming horrors emerging from hiding to do terrible things unto them is “what do we know about this creature’s strengths and weaknesses?”
Some of this knowledge is hard-won, and might only be known to the party if they’ve faced a particular threat before. But legends travel, and stories are told, of all manner of creatures. When asked, consider making an Intelligence check with the following guidelines. Only one check can be made per monster type unless you spend the time to dig up hidden lore.
Arcana will most frequently yield useful details about Aberrations (maybe), Constructs, Dragons, Elementals, and Monstrosities. However, some undead are created by powerful magic, and that might apply. Any creature with a reputation or history that makes them vulnerable (or not) to certain types of magical damage might have that known by making an Arcana check.
History will be useful for any type of creature that creates a civilization with a legacy and memory. Mostly Humanoids and Giants – but if a dragon decides to set herself up as Queen of a far-flung land, that territory’s stories, legends, conquests, and defeats will become part of the lore of the world.
Nature will tell for Beasts, Fey, Giants, Humanoids, and Plants. Some long-ago creations that have flourished and become part of an established ecology might also qualify (‘oh, the falcon-bear is native to these parts!’).
Religion will give some details about Celestials, Fiends, and Undead.
Take care to consider a character’s backstory when deciding what skills to use. A necromancer would certainly know about the dead and undead using his Arcana skill, while a cleric or paladin might have the same knowledge through Religion, and a bard via stories and songs passed down with the History skill.
Finding information about a particular creature is a matter of sifting through books, stories, rumors, and experience to determine what is known. Details such as overall appearance and behavior – such as determining what kind of monster is actually trying to eat you – might be an Easy or Medium task. Revealing tactically useful information should be harder (Medium to Hard), and doing that in the heat of combat should increase the difficulty level by at least one! The GM might give out one fact for making the roll, and more for each 5 points by which the ability check is made, or some other rationing of tactically-useful data.