here’s how I do knowledge checks

Make a knowledge check DC 15. for every 5 points over 15, give them one extra piece of useful knowledge. if it is very obscure, make it dc 20 or higher. some things they will not be able to know, such as what a monster is and its powers, but if they are confronted with something opaque, the knowledge check is a good tool. i like this because it can give them a lot of helpful information at once if they roll well, gives me discretion to tell them what i think would be helpful and useful, keeps them from having to play guessing game, but still has room for flat failure.

Example: “what did I know about that magic barrier preventing us from leaving the astral plane.”

wizard rolls a 22. 

me: “what kind of thing do you want to know?”

wizard: “well, can i still make sub dimensional planes with magic?” this is something he could figure out through trial and error, so the knowledge check is a useful tool to bypass that. but he rolled a 22, so he gets two pieces.

“yes, sub-dimensional planes can still exist. it seems likely the barrier only traps this portion of the astral plane (meaning, the dungeon they’re in) so if you can get out, you can probably escape.”

i like to ask them what they want to know. helps guide the conversation. 

i like to keep the world theyre in mysterious. sometimes there is no way to simply know what is going on because they are experiencing something new or unknown. often, the bulk of the game is trial and error, or learning through failing. 

“whats this floating giant cat in the next room?”

“you have no idea”

cat opens mouth really wide, revealing skull, shoot shoots eye-beams, turns you to marble. see, a knowledge check would just ruin it.

also, sometimes i will give them a piece of knowledge even if they fail, if the knowledge clears something opaque up. maybe a note i wrote to myself explaining the situation, that actually has no intelligible way to communicate. they puzzled over a small floating moon for a while, a wolfman was worshipping it but it didnt seem to do anything, and eventually they resorted to a series of knowledge checks to see if that helped. 

failed at that too, so:

“well, it appears to be a spirit of some kind. but that’s all you know.”

them: “a spirit???”

not exactly helpful per se, but gives the world a little more depth than just, “dont know what that is and it doesnt matter, so lets move on”

i like knowledge checks. a useful roll.