Kind of, yes. If it's not a coral snake, and doesn't have a triangular head, it's most likely fine. But the flip side is not true. Many non-venomous snakes have that head shape, and get killed because of it. Also, the cottonmouth does not have as pronounced of a pit viper head, as some of the...
Yes, they absolutely will. I had the biggest timber I ever saw in my life come right up to my coops, last year. Chased him off, and he was back in less than 5 minutes.
Rat snakes can get very large and sometimes will attempt to eat something far too large for them. They can very easily kill one of your hens. Snakes are immensely strong, and it would only take a minute for him to strangle a chicken.
Not true. Coral snakes have a slim head, and neurotoxin venom. Brown water snakes have a triangular head but are harmless. Not sure where that myth originated. It is true that pit vipers have a triangular head head, and most people think of rattlesnakes when someone says "snake!".