In the comments for another question it was implied that Monster Hunter (Ranger feat 1) was good, because it lets you identify a monster as a free action.
Knowing a monster's AC, saves, resistances and vulnerabilities would indeed be great, but I could not find any such thing at Recall Knowledge's description (CRB p239).
Is there any guideline on what I will learn, or is it completely up to the DM?
Best Answer
In the Game Mastering chapter, on page 506 of the Core Rulebook we get this information:
It is still left somewhat to GM decision what to share, but the intention is that you can learn one piece of information with one check, or two on a critical success. About what this "piece of information" is we get two other examples in the player section, on page 239:
These are really close to mechanical details, and I imagine this could mean a reveal of an exact numerical descriptor, such as AC or resistance, depending on play style. In most cases something like "You know it has a high Fortitude save" might suffice, as whether it has +20 or +25 might not change your decisions at all. The rules seem silent on how exact the information should be.
Also relevant is what we can read on repeated attempts to get more information, also on page 506: