In Xanathar's Guide to Everything near the bottom of the first column on page 82 is a table for Herbalism Kit Activities. One of the Activities is named "Identify poison" and requires a DC 20 to be successful. Since a DC 20 check is a bit high for identifying a poisonous plant (assuming the user has proficiency), I'm trying to determine for what types of poison this Activity is applicable.
My impression is that the DC 20 check to "Identify poison" may have been in the context of determining whether a Potion is a Potion of Poison. But in the case where it is implied that a user's experience making antitoxin (for those with proficiency) has given them insight on poisons, would the activity apply to all poisons, or just plant poisons? (Some poisons are derived from creatures or magic.)
(See this related question that clarifies that the intended use is indeed to identify poison, and not a potion, because except for a potion of poison, those can be identified by taste).
Question: Does the Herbalism Kit Activity "Identify poison" apply to all poisons or only plant poisons, and can it be used on a Potion to determine if it is a Potion of Poison?
Here is my reasoning for use on Potions of Poison:
None of the Herbalism Kit Skills indicate a thorough knowledge of poisons in general, but (for those with proficiency) the Special Use for the Herbalism Kit "Identify Plants." states:
Identify Plants. You can identify most plants with a quick inspection of their appearance and smell.
This indicates that identifying a poisonous plant is trivial in most cases (when the user has proficiency). This also suggests that the Activity "Identify poison" with a DC 20 may not be for identifying poisonous plants.
However, the Arcana skill description of the Herbalism Kit states:
Arcana. Your knowledge of the nature and uses of herbs can add insight to your magical studies that deal with plants and your attempts to identify potions.
Since identifying potions in 5E is trivial, it seems unusual to roll an Arcana check to determine what kind of potion it is. However, in the context of determining if it is genuine or a Potion of Poison, a DC 20 appears to be a reasonable check.