You can tell when your dog is dreaming by watching him. When he first falls asleep, his breathing will become more regular as the sleep becomes deeper.
When the dream starts, the dog's breathing becomes shallow and irregular. There may be occasional muscle twitches, and you can actually see his eyes moving behind his closed lids if you look closely enough. The eyes are moving because the dog is actually looking at the dream images in the same way that he would look at real objects in the outside world.
Not all dogs dream equally. It is an odd fact that small dogs dream more frequently than big dogs.
A dog as small as a toy poodle may dream once every 10 minutes, while a dog as large as a mastiff or an Irish wolfhound may spend an hour and a half between dreams.
Just as with humans, the amount of time dogs spend dreaming depends on age.
Young puppies spend more time dreaming than adult dogs.