Start with a field guide on birds for your region. Good to know what is around anyway. Not all hawks are a threat to all chickens under all situations but eventually the more problematic species will visit. Most methods for keeping hawks away are either impractical, ineffective, expensive of some combination of those. You can make hawk's job more difficult by having cover available, having larger chicken breeds, adult chickens and even roosters can help. Dogs can be handy although can be problematic in their own right. You can also restrict free-ranging to periods when you are outside. The effective measures do not work 100% of the time but they can come close and can be more cost effective than some enclosure options that simply keep chickens and raptors apart at the expense of free-range keeping.