It has been shown that people who make a habit of flossing have cleaner, more pleasant smelling mouths. The floss removes the buildup of bacteria from between the teeth. Bacteria lodged in the mouth is a very common source of mouth odor. Unfortunately most of the techniques, like the most popular blowing into your own cupped hands, are not effective. The human body becomes so accustomed to its own smell that it doesn't perceive it as being objectionable.
Some dentists have a piece of equipment called a halimeter.
It measures the amount of sulfur compounds that exist in your breath which translates into bad breath to one degree or another. Without making a trip to the dentist here are a few simple tricks to try to test your own breath. Run a piece of unflavored white dental floss between your upper and lower back molars. Smell the floss in about 45 seconds. Next - hold the tip of your tongue with a clean washcloth, while rubbing the back portion of your tongue with another white washcloth.