It's broad, but one of the biggest indicators that something isn't right with your pup is a departure from their "normal" behaviour. If your dog starts engageing behaviour they've never done before, or just doesn't have that same spunk, it could be a marker of depression. "Dogs can exhibit symptoms of unhappiness in a variety of ways," said Kayla Fratt, certified dog behaviour consultant and owner of Journey Dog Training. It's hard to predict how your dog will respond to stress, but the biggest thing to look for is a change from "usual" behaviour, which varies from one dog to the next.