For example :
You are dealt a card, face down, at a casino. Before you turn it over to look at it, the card you have been dealt is _______ [decided, but unknown].
For example :
You are dealt a card, face down, at a casino. Before you turn it over to look at it, the card you have been dealt is _______ [decided, but unknown].
Concealed would fit your criteria. For example, the dealer concealed the cards by placing them, face down, on the table. Other words include, unascertained, hidden, undisclosed, undetermined, undiscovered, etc...
You are dealt a card, face down, at a casino. Before you turn it over to look at it, the card you have been dealt is unrevealed.
M-W:
unrevealed
adjective
: not made known or opened up to view : not revealedIn a livestream event today, Blizzard revealed the final handful of unrevealed cards from its upcoming Hearthstone expansion, The Witchwood, which launches April 12 with 135 new cards.