You will know when a dog barks if you look at it. I mean, to bark is to open their mouth...
Otherwise, if a Deaf person happens to not pay any attention to the service dog, the service dog will tug for attention. I will know when my dog barks, especially with a leash on because I can feel the vibration via the leash. (and it is not a service dog...)
They are trained to acknowledge when their Deaf owner isn't looking and how they can get their attention. 
Tug on pants, touch on the owner's legs with their paws, jump up on the legs, et cetera...
PRETTY COOL, EH?
