So I have been trying to figure out the problem of when to call for a Perception check vs. an Investigation check. I have read over [this question](https://rpg.stackexchange.com/questions/47274/when-is-each-skill-used-when-searching-for-hidden-objects-like-traps/87081#87081%5C) extensively, but there is no accepted answer, and the most upvoted answer states that both perception or investigation could be used.

**I am looking for a *simple* RAW way to decide whether the player is rolling Perception of Investigation**, i.e., exactly **when** does the perception apply and when does the investigation apply?

An example of the kind of answer I'm looking for is: "Investigation is only used when the PC has a clue about whatever they are looking for. Perception is used when the PCs don't know what they're looking for." 

The goal of this answer is to allow me, the DM, to quickly decide whether Perception or Investigation is being used **AND** to be able to explain my reasoning to my players.

Bonus points if the uses of Investigation don't paint it into an obscure corner or rely on first making a perception check.