Scouting
Still being decidedLook at what somebody has before you attack them.
Not settled yet, so expect this to change.
Sending an Appraiser
Appraisers come from the Appraiser's Office and go and look at another player's city. The cheapest report tells you what they have in store. A dearer one tells you some of what is in their decks.
Hiding from one
Cards hanging in your gallery can be seen by anybody who scouts you, so hanging your defending deck tells people what you have. Keeping those cards in storage hides them, and makes your city less happy.
Warders are trained at the Art Gallery and stop Appraisers getting in. You cannot fake a deck you do not have, only hide the one you do.