Good suggestions from mimi and I would add that if your dog is of the long/floppy-eared variety be sure to check for ear infections. They are common and easily treated. If the dogs coat has a waxy or oily shhen and feel to it, another fairly common culprit can be treated and that's seborrhea. There are number of shampoos especially for this condition that will help and, over a period of time, usually eliminate the associated odor. Our Golden has this problem and a bath about once amonth with this type of product has just about cured it.
Smelly breath is a problem if it's more than just normal dog breath. Check for gum infection and redness. (Uneutered males have their own distinctive odor, but that source is readily apparent)...