When making decisions for which skill to apply to an on-the-fly knowledge check, I have some difficulties coming up with which skill rolls to ask my players for.
It seems strange to me that there is Arcane power, Divine power, Psionic power, etc... relating to magical powers and the like, but when making a magic-related skill check, there is only Arcana?
I house ruled allowing Religion, Nature, and Heal to identify or interact with corresponding ritual types. Also, I suggest using some creativity when it comes to "magical phenomenon"; the entire D&D world is magical after all. If the PCs are examining some runes that the wizard detects magic on to determine they are some kind of crude magical sensors, perhaps the dwarf trained in Dungeoneering can recognize these runes as poison-to-depth readings used to alert miners in the event of breaking a poison gas pocket, and the elf trained in Perception (or an auto-success for a Linguist/bard) notices that there is a hidden pattern among the runes - a trap?
Also I know that each monster's type relates to which skill the PCs need to role in order to discover information about it, but have yet to find a good table/chart describing this and the type of information that is provided on which DCs.
My cheat sheet has a very clear chart ( as does the Rules Compendium).
http://www.enworld.org/forum/d-d-4th-edition-discussion/307923-d-d-4th-edition-dm-cheat-sheet.html
Then I come to the part of 4E that suggests, "if the PCs need to know the information, give it to them." Why even have them make a knowledge check then? But if its information that they don't entirely need, its only thematic in essence, and therefore it should just be given to them also?
My approach is to have a very small amount of "need to know" information - this is the stuff that sets up the premise of the game, leads to a prepared adventure, or develops an individual PC's story. Obviously if you run more linear/path games there is more "need to know" than in sandbox games.
With "need to know" information there is no roll involved, there is no skill involved, the PC's just learn it. IME experience such information is best conveyed multiple times thru a "show don't tell" model. That increases the chances of players paying attention and remembering.
Then there's everything else, which is "optional" info, and encompasses the vast majority of information the PCs may seek out. Generally if a player asks a story question that's a good indication for a knowledge check, versus top-down into initiated by the DM which is generally "need to know."
With "optional" info, a knowledge skill is usually involved. Often this will mean a roll, but sometimes "taking 10" (or a passive check) makes more sense. This can be conveyed simply from DM to player (eg. "you know the duke is alleged to have incestuous relations with his aunt"), thru a plot exposition NPC, a book, or anything really.
So that's the difference.