The next sound you hear

Last night I was in the backyard with Roc, who for some reason, even after four years, is not really secure on the whole housebreaking thing, which means when he suddenly runs out of the room where he's been peacefully napping you run after him. Unless you feel like cleaning up the mess later. Most of the time he's just desperately thirsty and running to the kitchen for a drink.

Of course dogs get used to routines, they train us very well that way. So after all this time, he gets up, runs out of the room and waits for one of us in the hallway. He knows we're coming, and he's a very social little guy, so he does the polite thing and waits and now we're stuck. We decided to trust him for a while, but he kept waiting in the hall. Like I said, he has us well-trained.

Anyway, I had him in the backyard and like the nine-pound protector of the universe he is, he heard another dog walking in the alley and starts yapping away. I tried to get him to quiet down, but then the other dog barked. It was very dark and the other dog was on the other side of a fence and barrier of bushes. I couldn't see the other dog at all. But I heard his owner say "Quiet," and we had a nice chat:

He: "What kind of dog do you have?"
Me: "Brussels Griffon. Is that a Shepherd?"
He: "Yeah, he's a German Shepherd"
Me: "Have a good evening."
He: "You, too."

So my question is, little dog person that I've been all my life - how did I know, just from its bark, that his dog was a German Shepherd?