No avocados or raw potatoes.
There are some things that they will probably decline, like carrots. They don't like hard things like raw carrots, but also don't like really mushy ones, either. They love greens, and reds come in second. My chickens love lettuce/spinach, apple cores, cucumbers, cooked potatoes, and broccoli (the tops and insides of the stems.) They're not fond of bananas or strawberry tops. Melons are a toss-up... they love them on a hot day, but if it's not too hot then the gooiness might turn them off.
Also, I've noticed that younger chicks might refuse something that mature hens love. Since chickens usually know better than humans what's good for them, I'd just let that be a good indication. My daughter wanted to give my mama hen and her baby some old corn cobs yesterday, but I didn't know if the baby's digestive system was ready, so we gave them to the grown hens.