Are Knowledge checks proactive or reactive?

[shilsen]

They are both, sometimes a DM asks for a character to make it and sometimes the player requests it to happen

What he said. I ask for Knowledge checks sometimes and players sometimes ask if they can make a check.

 

[ThirdWizard]

Players ask for it when they want to know something. Otherwise, I'd be calling for checks all session.

 

[TheAuldGrump]

It's no different to describing what a character sees or hears. Knowledge is automatic. You can't expect a play to know to ask "Can I make a spot check?" an "Can I make a listen check?" every time he turns a corner, or to ask "Can I make a sense motive check?" every time someone talks to him.

I'd ask them to make the check, not expect them to know to ask to make the check.

That is good for things that they don't need to mull over - but if something is relatively obscure they might need to think 'er over for a bit, whch is where the PC asks to make a roll. Which is why I say both.

The Auld Grump