I don't know if there's a rule, but when I play I tend to build the discard pile fanned so that everyone can see what's there. There seems to be no reason to turn it into a memory game and the game's decisions are more interesting as more information becomes available.
It is in the rules - p.11.
"All discarded cards remain in front of the player who dicarded them. Overlap the cards so it's clear in which order they were discarded."
If you are supposed to see an order, you must be able to see all the cards (and not just the top card).