When I visit my vet 

I noticed she always smells my cat's breath during the exam. There is a good reason she is doing this. A stinky breath once in a while is not cause for alarm but if your dog or cat has chronically bad breath, it could be a sign of a more serious condition.  
Call your vet if you notice one of the following.An ammonia or urine like smell.  This can be a symptom of kidney problems.
A rotten smell.  This indicate intestinal issues or gum or tooth disease.
A sweet almost sugary smell.  This often signals diabetes.  You will also notice your pet drinking a lot of water and losing weight.