101.4a

If an effect has each player choose a card in a hidden zone, such as their hand or library, those cards may remain face down as they're chosen. However, each player must clearly indicate which face-down card they are choosing.

Liliana of the Veil. When Liliana’s first ability resolves, first the player whose turn it is chooses a card to discard, then each other player in turn order chooses a card to discard, then those cards are discarded simultaneously. No one sees what the other players are discarding before deciding which card to discard.