There are several ways to tell if a dog is friendly or aggressive. A friendly dog will typically approach you with a wagging tail and relaxed body language. It may also lick your hand or nuzzle you affectionately. An aggressive dog, on the other hand, may growl, bare its teeth, or raise its hackles. It may also stand rigidly or lunge towards you. If you are unsure whether a dog is friendly or aggressive, it is best to err on the side of caution and avoid approaching it.