It's been shown in the Terminator series repeatedly that dogs can tell a Terminator infiltrator from a human being. Did the movie series or book ever explain how they could do that?
I don't know of it explicitly being stated, but it's easy to imagine means by which they could distinguish terminators from humans. One of the most obvious is smell; dogs have a sense of smell that is much more advanced than humans', and Skynet like doesn't/can't create terminators that that smell to dogs like humans. There are more speculative hypotheses, such as that terminators have mannerisms that are different from humans'. Or perhaps they emit ultrasonic noise.
Dogs being able to distinguish terminators and them barking at them are not quite the same thing, but it's not hard to imagine that a dog encountering an object that visually appears to be human but otherwise doesn't appear to be human would cause confusion and distress. Or if a dog is present for multiple terminators infiltrations, it would come to associate terminators with bad things.
Almost all dog owners feel that there is extrasensory perception in dogs. Research shows that dogs have electromagnetic intuition.
Diana L. Guerrero, who is doing research on this subject, in an interview; He says that each animal behaves differently before an earthquake: "They usually don't hide. Their behavior detected before an earthquake is howling, whining, barking, and a more intense attachment to their owner. They come and go between street doors, room doors and windows and stick to you like glue. Their personality and If you pay attention to their individual needs, you will be protected from a possible danger. The increase in the number of lost dogs and the number of dogs in shelters is a serious clue to you about a big earthquake."
I don't remember an explanation in the movie or the book. I think he may have made such an inference based on these circumstances.
I hope it was a good explanation. :)