Wondering whether or not you have bad breath is a common fear. There's even a term for it — halitophobia. One study found that 80 percent of patients visiting a particular dental clinic thought they had stinky breath (also known as "halitosis") when only 24 percent of them actually did [source: Sterer, et al.]. Such an exaggerated worry likely arises from the fact that we can't actually smell our own halitosis. As a result, we fear we're unleashing a gust of dragon breath every time we talk to someone.
There are reasons why we can't smell our own bad breath, but they are hypotheses at best. First up is the theory that we're overly accustomed to our own bodily smells. There is something to this idea. The same thing can happen with our living environments as well. Have you ever noticed that you can't detect your house's subtle smells until you come back home from a vacation? The same may apply to your breath; maybe you're just too used to it.
Another reason could be that our breath releases differently during talking than it does during breathing. When you're speaking, you're more likely to stir up the back of your mouth, which is where halitosis stems from.
The next idea is that our inability to smell our own breath — or the inside of our noses, for that matter — is an evolutionary development. According to this particular theory, it would overwhelm the nose to constantly pick up the smell of breath below it, and this would keep it from detecting other odors more critical to safety and survival. As a result, the mind learned to block out the scent of the nose's nearest neighbor.
Finally, a more recent assumption is that the smell is not potent enough to last through the process of exhalation and inhalation in one person. Therefore, any halitosis a person might have dissolves before his or her nose can detect it. However, if person A breathes on person B, person B can smell it because he or she is only having to inhale person A's breath, not exhale it first.
So how can you know if you have bad breath? Common methods include licking then smelling the inside of your wrist, or examining your tongue in the mirror to see if it's coated. If you prefer an official answer, you can order a breath testing kit or have your dentist perform a halimeter test.