According to the Player's manual"[Y]you can use [Knowledge skills] To identify monsters and their special powers or vulnerabilities. In general, the DC of such control is equal to 10 + the HD of the monster. A successful check-up allows you to remember some useful information about that monster. "(78) And there are no other guidelines, that is, there is a great sinister in general pressed there to allow the DM to manipulate, of course, but otherwise the DM is alone.
That changes, I think (and comments can correct me completely if I'm wrong), July 2006, when Monster Manual IV Includes a story entry for each creature. At ManualThe introduction under the title Lore, says of the Player's manualThe use of knowledge skills to identify creatures.
The specific information of the creature, such as its type of resistance to damage, abilities similar to spells or immunities, comes with the result of high DC control. (6)
Then, without further ado or explanations, each of these tradition entries, instead of being based on the creature's HD, is based on the creature challenge classification (a system later adopted by Pioneer), with the DC from 10 + CR, a change that makes sense to a certain point but leaves cases of corners without direction as advanced creatures with template.
I did a little research and there was no Design and Development column (yes, they're really called that, Oh, crazy Wizards!) Monster Manual IV, and Dave Noonan does not mention the entries of the tradition in his articles here and here about the design and development of the Monster Manual V, so I do not know the reason behind the change from HD-based identification to CR-based identification. (By the way, these are the last 2 published Design and Development articles, which is a shame). (In addition, I was disappointed with the elimination of the Environment and Treasury lines in the last entries of creatures, but that is another problem.)
The gnoll entry in Monster Manual IV (67-71) presents CR gnolls of 3, 4, and 6, but at the same time provides mathematical knowledge results based on the gnoll of CR 1. The design intent seems to be that when an advanced or ranked creature is found , the CR of the base creature is used to gather its knowledge. Templates appear, based on templates played with Lolth (MM4 94), of God's blood (MM5 66), and ghost (MM5 130) – to require a Knowledge Ability test with a DC of 15 + the new CR of the creature or a DC of 15 directly to identify the template. That's single for the template, however; the identification of the underlying creature is a verification of separate Knowledge abilities.
An effort was made on the message boards of Wizards of the Coast to compose entries from the tradition of the creatures that lacked them. The results are here, although there is a more friendly format with the tables here.
"But what about the dragons?"
Drow of the Underdark gives us the deep dragon (114-7), the first dragon I could find afterMonster Manual IV that has age categories Y An entry of tradition. The DC Knowledge test for the deep dragon is 15 + the dragon's RC, which implies that the dragons of different CR are different creatures, so they require separate Knowledge tests.