How do I now if I have a fox hybrid?

Foxes are Vulpine, dogs are canine.

they cannot breed.

Fox does not mate up with dog. Terrier howls when frustrated. Kind of funny. Terrier likes his territory and his "stuff" and will hide & protect it.

I would offer up that you have a terrier of some kind. Train him to resist barking/howling by a sound greater than him. (Pennies in a can, boat horn, or gun fire)

We have a 20# rat terrier that freaks out at every motor.

There has never been actual solid proof of a fox-dog hybrid. Only reports. It may be possible but is very, very, very unlikely/rare.

Oh and number of chromosomes is irrelevant. Since horses and zebras can breed.

It's highly unlikely occasions of dog/fox hybrids are extremely rare we do know that its possible but almost unheard of. The only way to tell would be genetic testing which your vet could do.

Dogs and fox are two different species and can not breed, as their genes and Chromosones are different

Foxes can't mate with dogs. They are more similar to cats than dogs.