Being in the presence of an unknown dog can be quite intimidating. Most of the time, the dog will be friendly or at least non-threatening. However, if a dog is aggressive or prone to biting, it is helpful if you are aware of the signs.
You should never approach a dog with which you are unfamiliar, but being able to identify a dog who could attack you is essential to knowing how quickly you need to get away from the situation. The ASPCA explains that not every dog will act the same way or give any warning before biting, but there are some telltale signs that are common.
Dog moods
Dogs who are most likely to bite will generally react in an aggressive, scared or anxious manner. These dogs feel unsafe or threatened by you and feel they must protect themselves. They may also not know how to react to the situation except through biting, which is the natural protective instinct most dogs have.
Signs to watch
Aggressive dogs will often have a very stiff posture. The tail may be straight and still. The dog may try to look bigger by taking a wide stance and staring at you. The dog may or may not growl and show its teeth. The posture is more important to note and what tells you to get away.
Scared dogs generally want to try to get away, but they may not be able to find a way to retreat. They may curl up or crouch. The tail may go between the legs. The dog will generally stay still. It may growl.
Anxious dogs may pace or shake. They may lick their lips a lot and avoid eye contact. It might start barking or growling as well.
Any of these signs from a dog means you should not approach it. It is not feeling safe or friendly, and a bite is likely to occur.