All I could find about this subject is in Detecting a Hazard:
When characters approach a hazard, they have a chance of finding the trigger area or mechanism before triggering the hazard.
Anyone who succeeds becomes aware of the hazard, and you can describe what they notice.
But what does this mean? Do you only know that there’s a trap, or the trigger of one, in that square? Do you know the whole trap statblock? Something else?