Eric Brosius wrote:
Yes. If there is only 1 card left in a stack, you cannot take it with the Observatory. If there are 2 or more cards left, you can take the top 1. Hence, it must be legal to count how many cards are left in a stack.
Well, actually this only proves you can know when a deck contains just one card (you should draw the card using observatory without looking at it, if you see no more cards are in the deck then you put it back in the deck face down).
That said, we allow players to count the cards, too. Actually, in the italian translation of the rules we learned the game from, there's a note explicitly saying that any player can count the cards anytime (don't know where that note came from).