I do monster knowledge checks like this:
Applicable skills: Arcana (for non-natural monsters), History, Nature, Religion (for undead), and Survival
If the PCs clear the following DCs, they know...
DC 5 - the name of the monster
DC 10 - how it typically attacks
DC 15 - some special resistances, if any, all possible attacks, and one or two ecology facts
DC 20 - all resistances/immunities, if any, special effects from attacks, and three or four ecology facts
DC 25 - essentially everything about the monster, including reach of attacks, and which attacks cause more damage, and the entire ecology entry
I never reference anything in game terms, so no HP or damage dice, but I'll make comparisons with other characters/monsters. E.g., "It looks sturdier than the Fighter," when the fighter has 100+ HP.
I'll also adjust the above DCs depending on how common or rare the monster is. Goblins/Orcs/Bugbears, etc. would be easier DCs, while some abomination from another plane might have more difficult DCs.