Not only are they great guardians against sneaky people, dogs (and all animals) are very sensitive to body language, smell, and a person's 'aura.'

Every Thanksgiving we have a whole bunch of people and dogs. These dogs are well socialized and love everyone! One year the democrat bit of the family shows up. All dogs immediately put up their hackles and start barking and growling. It takes quite a while for them to calm down, but they never go near those people.