Crthuky

To kill a Lich the PCs must first destroy the lich's phylactery. From the Monster's Manual we know the basic physical description of this item:

A phylactery is traditionally an amulet in the shape of a small box, but it can take the form of any item possessing an interior space into which arcane sigils of naming, binding, immortality, and dark magic are scribed in silver.

It is possible for a given item to meet the physical description of the phylactery but not yet be activity used. I.e. The lich has not yet performed the ritual to bind their soul to the item.

What methods exist for the party to determine if an item is a phylactery?

Presumably the Identify spell will do the job but is that the only way? How long does it take for a given item? Particularly can a PC check multiple items at once or do all methods require a one by one approach?

For some context imagine the following scenario.

We know that liches are very protective of their phylactery:

Because the destruction of its phylactery means the possibility of eternal death, a lich usually keps its phylactery in a hidden, well-guarded location.

Given that liches are typically highly intelligent suppose they create a room to store their phylactery. After fighting past all of the physical and magical protections on this room the party is met with a room where dozens of objects sit on pedestals.

Each of these items meets the physical description of a phylactery, but only one is the true phylactery. In order to defeat the powerful lich the party must find the correct object and destroy it before the lich can complete its evil plan.

How can the players find the true phylactery as quickly as possibly?

The main question is asking for any possible way of determining if a given item is a phylactery. If there are multiple ways, preference is for the fastest (in terms of action economy) followed by lowest resource expenditure (spell slots, 1/rest abilities, gold) and finally by earliest availability (by class level).

A lich is exceptionally intelligent and careful, so this is likely to be a challenge. There are two main approaches:

• Use divination magic


• Use the unique properties of the phylactery to rule out which items cannot be the phylactery

Divinations

• 
  Commune (5th level) allows you to ask three yes or no questions. The optimal way to use this is what is known as a binary search. Suppose for example that there are 12 phylacteries:
  
  
  • Place the phylacteries around you in a clock pattern, with yourself in the middle, facing between 12 o'clock and 1 o'clock.
  
  
  • "Is the lich's phylactery one of the six to my left?" This will eliminate half of the possibilities.
  
  
  • "Is the lich's phylactery one of the three in front of me?" This will eliminate another half of the possibilities, leaving three remaining.
  
  
  • "Is the lich's phylactery the one I am touching now?" This has a 33% chance to get it right. If it fails, one of the remaining two must be it.
  
  


• 
  Divination (4th level) lets you ask one question, but you can use as a supplementary method if your commune failed to determine the exact phylactery and there are only two options left. Casting commune twice in one short rest has a chance to fail, but commune plus divination does not.


• 
  Legend lore (5th level) tells you facts about an item you name. The information you gain may be enough to narrow down the possible phylacteries.


• 
  Contact other plane (5th level) lets you answer five questions, but it's a dangerous spell to cast.


• Obviously, wish (9th level) or a cleric's Divine Intervention.

Properties of a phylactery

• If the lich is killed, the lich only reappears within 5 feet the true phylactery. Place the phylacteries far apart, kill the lich, and see where the lich re-appears.


• A phylactery is typically not easy to destroy without some special method or equipment. Anything that is easily damaged can probably be discounted.


• A phylactery contains a soul, and also imprisons souls. If you had some way to detect souls or the movement of souls, you could use this property to detect the real item.


• A phylactery contains certain arcane sigils inscribed in silver upon its interior. Any item that doesn't is fake. The limit is that many phylacteries are opaque and nearly impossible to break, and therefore difficult to see inside without something like a ring of x-ray vision.