How do you determine if berries are poisonous?

To determine if berries are poisonous, you should consult a reliable source such as a field guide or a knowledgeable expert. It is important to accurately identify the berries and cross-reference them with information on known toxic berries. Additionally, you can perform a skin test by touching a small amount of the berry to your skin to check for any adverse reactions. If in doubt, it is best to err on the side of caution and avoid consuming unknown berries.