Horns of Doom
Knowledge skills may be used to identify monsters, as well as understand their powers/weaknesses, according to chart on page 180 of the PHB. Knowledge of different types of creatures depend on different skills:
Arcana: Fey, Elemental and Shadow creatures
Dungeoneering: Underdark creatures, Aberrants
Nature: Natural creatures
Religion: Immortal and Undead creatures
Where creatures have overlapping keywords, I will make a judgment call on how or if different skills apply for knowledge of that creature.