The one I read before went a bit further...
A guy goes into the vet's with his alsatian, which he says is having trouble waking up. The vet examines the dog, puts on a grave face and explains that the dog is dead.
The man demands a second opinion.
The vet brings in two animals from the next room - a ginger tabby, and a black labrador. The cat walks around the alsatian twice, looks up at the vet and offers a plaintive meow.
The vet tells the man that the cat thinks his dog is dead, too, but the man demands another opinion.
So the black labrador goes over to the alsatian, sniffs it, lowers his head and growls softly.
The vet tells the man his dog is dead, and both the cat and the labrador agree. The man finally accepts the diagnosis and asks the vet how much he owes him.
"£650" says the vet.
"£650?" says the man. "How on earth can you justify that?"
"Well," says the vet, "...it would only have been £50 but you insisted on the cat scan and the lab tests..."