In another thread there was a general concensus that you can't ask the number of the CR but you might be able to ask for a range of how tough the creature might be. This is fairly consistent with my own houserule, but I'm realizing I've only ruled on a couple questions. Does anyone have a list of all the things you can ask for on these checks?
I've always read...
Check: Answering a question within your field of study has a DC of 10 (for really easy questions), 15 (for basic questions), or 20 to 30 (for really tough questions).
...and...
You can use this skill to identify monsters and their special powers or vulnerabilities. In general, the DC of such a check equals 10 + the monster's CR. For common monsters, such as goblins, the DC of this check equals 5 + the monster's CR. For particularly rare monsters, such as the tarrasque, the DC of this check equals 15 + the monster's CR, or more. A successful check allows you to remember a bit of useful information about that monster. For every 5 points by which your check result exceeds the DC, you recall another piece of useful information. Many of the Knowledge skills have specific uses as noted on Table: Knowledge Skill DCs.
As separate things; i.e. you cannot ask questions about monsters, you merely roll against the appropriate DC and the DM presents you a summary of what you know.
Edit: That being said, and this being the homebrew section, I give information first on the most iconic abilities of a creature (a Troll's regeneration and his weakness to fire, for instance) and then at each +5 the DC is beaten by I give one piece of information from each Special attack, special defense, special quality (including auras), DR, Resistances, SLAs and Spells (for things like faerie dragons) and Spell Resistance. I'd have to check my houserules document, but I think that is everything.
Oh, I thought the mechanic is that the player makes a roll and if successful you get the baseline traits. If you succeed by 5 or more you get one additional question such as "what's its worst save; does it have a weakness" etc. Maybe I stole this from PFS games?
The way I've handled it was that creatures have a baseline DC of 10 + CR to identify them, which gives their name and most identifiable feature (they use mind control to catch prey, etc.). Failing by less than 5 will give the creature type or other vague information, and succeeding by more than 5 will yield all relevant info.