People tell you. Most of the time other people will tell you or offer you mints and gum a lot. You can't smell your own bad breath. Proper brushing and flossing daily and seeing the dentist every 6 months will go a long way toward curing your halitosis. You must floss to remove food debris and plaque, brush the tongue thoroughly. The tongue is like a carpet for bacteria to thrive on. Halitosis has multiple causes.
Take a whiff... Halitosis is just a fancy word for 'bad breath'. If the breath smells bad, there you go...Positive diagnosis. The real question is diagnosing why someone has halitosis. It could be from sinus problems and post-nasal drip, it could be from gum disease, or from other conditions.
If one smells odor. Halitosis (bad breath) after certain foods or drinks occurs for several reasons. Chemicals from some foods/drinks diffuse into the flesh in the mouth and throat. These chemicals (odors) keep coming out into the breath for many hours. Some foods/drinks cause odors when in the stomach. Stomach odors will come up through burps, and even without noticeable burps. Some substances stick to the teeth.