The rulebook says on page 17, first column:
"Because all secret cards/tiles have numbered codes or bear additional marks or art, the players may sometimes identify a card/tile by its back before it is actually revealed. This is not a problem, and it is intentional."
The rulebook also says on page 10, first column:
"The top card should always be the only visible card of the Turn deck."
I thought, because of the quote from page 17, that it was ok to look through decks like slumber, influence, or turns, just looking at the backs of them, to see when something is coming up. How many moves on the map until we reach the delta phase? How many turns must I evade the wraith in order to defeat him on a turn with the best loot that I need the most? How many cards do I need to draw to get back to having my character's power on the back of the top card? These are questions I want to know and I thought I could just check. But then I re-read the rulebook again and noticed the page 10 verbiage... so now I'm wondering if I was wrong.
This is really be a more generalized question of whether we're allowed to look at the backs of upcoming cards in a stack... for turn, influence, slumber, or any other decks.
We are definitely not allowed to look at future turn cards (unless you have an action that allows this).
The rest here is my take, not sure if this is stated in the rules.
Generally, you are always allowed to count the number of cards in a deck. So I think you can find the number of slumbers before the next delta phase or how many cards until the next card with a dreamer ability.