How much does the Identify spell tell me about a Deck of Many Things?

Question

When cast on a magic item, the Identify spell tells you, among other things, "its properties and how to use them". If I cast this spell on a Deck of Many Things, how much do I learn about it? Certainly I learn the mechanics of how to declare and draw from the deck. But what else do I learn beyond that?

For example, which of the following do I learn?

• How many cards are currently in the deck
• How many cards were originally in the deck
• Exactly which cards are currently in the deck
• The effect of drawing each card in the deck
• The stats of an Avatar of Death

Answer 1

Ask your GM; "you learn its properties and how to use them"

The quote above is from the identify spell.

"Property" is defined as:

An attribute, quality, or characteristic of something.

But the interpretation of this phrase would be entirely up to a GM because what count as "an attribute, quality, or characteristic" of a Deck of Many Things is up to interpretation.

This is effectively identical to the section on "Identifying a Magic Item":

The identify spell is the fastest way to reveal an item’s properties. Alternatively, a character can focus on one magic item during a short rest, while being in physical contact with the item. At the end of the rest, the character learns the item’s properties, as well as how to use them.

Showing again, that the interpretation of this phrase is up to the GM (this wording is found in the DMG after all).

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My personal interpretation:

You learn a property if and only if you can actually use it.
You can use a property if and only if you can specifically choose for it to occur.
Therefore you do not learn what each card does because you cannot select a specific card.
You do not learn how many cards are ordinarily in a deck because this is not something you have control over.
You do not learn which cards are in the deck because this is not something you can specifically choose to have happen (which card(s) are in the deck is random).

Answer 2

You can know the cards in the deck, but not what you'll draw

New information on the Deck has been published since you asked this question. The Book of Many Things gives a more detailed explanation of how the deck works under The Deck in Play / Handling the cards:

The Deck of Many Things has the power to transform anyone who draws from it, but to trigger its effects, the individual handling the cards must state their desire to draw from it, as well as the number of cards they intend to draw. Otherwise, the deck's magical powers lie dormant. Anyone who identifies a deck learns this about the deck in the process, per the rules for identifying magic items.

This means that under normal circumstances, characters can handle a Deck of Many Things safely. A character who finds a deck can look at it, spread the cards out, and otherwise manipulate the cards without triggering their effects.

This would indicate that you can look at all the cards, even without an identify spell, and therefore know what cards make up the deck. So this is clearly a knowable property that identify should be able to discover. However, what you will not learn is which card you will draw when you draw, because the deck states (DMG, p. 162):

Before you draw a card, you must declare how many cards you intend to draw and then draw them randomly

That is, even if you as a character looked at all the cards before, and ordered them to put all the good stuff on top, once you draw, the card you will get is random. It's magic.

Partial decks

The solution to the problem of how that works if the players remove the negative cards first is simple: only when the whole deck is assembled, can its normal magical function be exercised. For example, the book suggest you can let players find individual cards separately first, and have these cards have some minor magical effect by themselves, but not the effects described in the deck. Then:

Eventually the heroes assemble the entire deck, which they can then use in the traditional manner, drawing transformative cards from it in a fitting climax to the campaign.

Stacking the deck

The Book of Many things also explains many other ways to use the deck, including that it is up to the DM to pre-determine the order in which you will draw cards "randomly", as long as it fits to their vision for the campaign, and says, addressing the DM:

You can ensure certain outcomes by stacking the deck with cards placed in a specific order.

So, you should be able to learn the number and names of the cards in the deck. When it comes to how much detail about the deck and effects of each card you further can learn, as this answer states, the rules do not specify further how to interpret what is included in "the item's properties" that you learn from identify, so that will be up to the DM.