API HasSoulstone

Returns whether or not, and how, your character can self-resurrect.

text = HasSoulstone()

Returns

text: string or nil - the type of self-resurrect available to your character (known values are "Use Soulstone", "Reincarnate", and "Twisting Nether") or nil if none are available

This function also returns nil if used while your character is alive.
The return values from this function are (presumably) localized.
The text returned by this function is used in the default Blizzard UI as part of the popup that appears when your character dies, to determine whether the second button should be shown, and if so, what text should be shown on it.
The "Twisting Nether" self-resurrection is a proc from the item [Darkmoon Card: Twisting Nether].