So you want to know how to tell if a horse is friendly? Horses communicate with body language, so once you know certain signals to watch for, it's easy to tell if a horse is friendly. To tell if a horse is friendly, watch the ears and the tail.
A friendly horse will perk his ears forward, interestedly. An unfriendly horse will pin his ears back flat against his head
A friendly horse will usually approach a stranger casually. An unfriendly horse will many times back away or cower.
A friendly horse will have a relaxed tail, perhaps casually swishing flies. An unfriendly horse will anxiously twitch or wring his tail.
With these few signals you can easily tell if a horse is friendly or unfriendly. Remember, friendly or not, to always ask permission before petting a horse. And remember, some horses you can tell by body language are friendly- can still bite when looking for a treat.