While roleplaying, you can generally roll a skill check whenever you think it's appropriate.
I will also post blocks with a skill and target DC, and indicate whether that DC is for active or passive use of the skill. If it's a passive target on a knowledge skill, usually that means that you either succeed or fail, and can't make an active check. If you can make an active check, I'll indicate that.
Finally, I will nest skill check blocks inside each other. What this means is that the inner skill blocks are "gated" by the outer ones. If you don't succeed at whatever check you need for the outer block, you can't make the check for the inner ones. But if the inner block says "same check", don't make a separate check, use the same check you used for the outer one.
This is a bit of an experiment. The idea is to simulate general knowledge, which anyone with a certain level of skill would know, then more specific tidbits of information that different individuals might or might not know - but if they didn't know the common knowledge, they wouldn't know the specific knowledge either.
Here's an example:
Ok, so the example is pretty complex; I probably won't use a real one anywhere near that crazy. But here's how it breaks down.
History +0 (passive 10): You know the stuff in the first block about the forge.
History +5 (passive 15): You also know the stuff in the second block about tender, and can make two active history checks.
First active check: if you get 15 or better, you know the story about Tender repairing Arek. If you get 20 or better, you know the story is false.
Second active check: if you get 25 or better, you know about Tender's favorite oil.
It's the honor system for not reading inside blocks you don't qualify for. Actually, I don't really mind if you do read inside the blocks, as long as you keep your player and in-character knowledge separate. It's up to you.