Do you ever wonder how dogs find criminals in a passenger ship? It's simple: by sniffing. They are called detection or sniffer dogs. Trained to detect people and substances, through their senses, such as explosives, illegal drugs and blood; almost always the sense of smell, which is two thousand times much stronger, and fifty times more sensitive than that of a human's. In addition to sensitivity, a dog's sense of smell is pricky.
It can discern a specific scent; meaning it can recognize cheese in a fish and vegetable market thousand miles away from where it is standing. Hunting dogs that search for game and search dog that search for missing humans are generally not considered dogs. However, there is some confusion with cadaver dogs, only trained to detect human remains.