Keeper of the Lens

• WWizards of the Coast2015-02-25
  Official Ruling
  Keeper of the Lens doesn’t stop your opponents from looking at face-down creatures they control.
• WWizards of the Coast2015-02-25
  Official Ruling
  Keeper of the Lens allows you to look at face-down creatures you don’t control whenever you want to, even if you don’t have priority. This action doesn’t use the stack.
• WWizards of the Coast2015-02-25
  Official Ruling
  Keeper of the Lens doesn’t let you look at other players’ face-down spells while they’re on the stack.

Rulings via Scryfall