Body language:
Calm: A calm dog will often have relaxed body language, such as lying down with its legs stretched out, sitting with its head resting on its paws, or standing with its tail held naturally.
Sad: A sad dog may have a hunched posture, with its head lowered and its tail tucked between its legs. Its ears may be pulled back against its head.
Facial expression:
Calm: A calm dog will often have a relaxed facial expression, with its eyes open and its mouth closed.
Sad: A sad dog may have a sad or mournful expression, with its eyes half-closed and its mouth slightly open. Its eyebrows may be furrowed and its ears may be pulled back.
Activity level:
Calm: A calm dog will typically be content to relax and stay in one place.
Sad: A sad dog may be less active than usual, and may spend more time sleeping or lying down. It may also show a lack of interest in activities it normally enjoys, such as playing or eating.
Vocalization:
Calm: A calm dog will typically only vocalize when it has a reason to, such as barking to alert you to someone at the door.
Sad: A sad dog may whine or bark more than usual, as a way of expressing its sadness or frustration.
Appetite:
Calm: A calm dog will typically have a regular appetite.
Sad: A sad dog may lose its appetite, or may only eat small amounts of food.
Interaction with others:
Calm: A calm dog will typically be sociable and friendly, and will enjoy interacting with people and other animals.
Sad: A sad dog may be less social than usual, and may avoid interacting with people and other animals.
It's important to note that dogs may display a variety of behaviors when they are sad, so it's important to look at the overall picture of your dog's behavior to determine if it's sad or just calm. If you're concerned that your dog may be sad, it's always a good idea to talk to your veterinarian.