there may be some that can harm chickens, u can look up which (if any) are harmful either on the website or another. Also, chickens are good at telling u wether they will eat it or not.....they might not like the flavor though.
Given free choice, chickens appear to not eat things that are bad for them. I suspect that mother nature has equipped them to make the right choices. If they are going to forage, they likely won't eat what they should not.
My chickens won't touch the basil, mint, and oregano in the garden. -lots of herbs must be unpalatable to chickens. They do like parsley. My birds do leave alone things that aren't good for them (such as the Jasmine and Lantana) and seem to forage on the things that are agreeable.