Chemistry

Chemistry is a skill in the game that is raised by mixing certain ingredients, creating herbal potions, and crafting deadly poisons. If you have a high enough chemistry skill (10) you can tell whether or not a food item is poisoned by hovering over the item name. 
Some items that are associated with the skill are: Poison, Charcoal, Berry juice, Apple juice, and Prickly Pear juice
To start leveling chemistry, mine coal, and create charcoal
At chemistry level 3, begin harvesting herbs and flax flower, to create herbal remedies
After creating a few batches, you should be at level 8 chemistry, which is when you can create poison using mushrooms
As mentioned above, chemistry 10 allows you to inspect food items to see if they're poisoned.