DM's Note
For my campaigns, when you roll a Natural 20 on a skill check roll, it is automatically considered a 30 (+ all relevant bonuses), as long as the PC has at least 1 rank in the skill.
For Knowledge checks, if the player wants to make a second, related Knowledge check based on new information learned from the Nat 20, there is a +2 circumstance bonus due to favorable circumstances. If that roll is also a Nat 20, the player can roll a third time for another related field. I, as the DM, will decided if the fields are related.