They often use some of several laboratory methods for diagnosis of bad breath:

Halimeter: a portable sulfide monitor used to test for levels of sulfur emissions (to be specific, hydrogen sulfide) in the mouth air. When used properly, this device can be very effective at determining levels of certain VSC-producing bacteria. However, it has drawbacks in clinical applications. For example, other common sulfides (such as mercaptan) are not recorded as easily and can be misrepresented in test results. Certain foods such as garlic and onions produce sulfur in the breath for as long as 48 hours and can result in false readings. The Halimeter is also very sensitive to alcohol, so one should avoid drinking alcohol or using alcohol-containing mouthwashes for at least 12 hours prior to being tested. This analog machine loses sensitivity over time and requires periodic recalibration to remain accurate. Gas chromatography: portable machines are being studied. This technology is designed to digitally measure molecular levels of major VSCs in a sample of mouth air (such as hydrogen sulfide, methyl mercaptan, and dimethyl sulfide). It is accurate in measuring the sulfur components of the breath and produces visual results in graph form via computer interface. BANA test: this test is directed to find the salivary levels of an enzyme indicating the presence of certain halitosis-related bacteria.
Food debris becomes trapped, undergoes slow bacterial putrefaction and release of malodourous volatiles. Food packing can also cause a localized periodontal reaction, characterized by dental pain that is relieved by cleaning the area of food packing with interdental brush or floss. Acrylic dentures (plastic false teeth) – inadequate denture hygiene practises such as failing to clean and remove the prosthesis each night, may cause a malodour from the plastic itself or from the mouth as microbiota responds to the altered environment. The plastic is actually porous, and the fitting surface is usually irregular, sculpted to fit the edentulous oral anatomy. These factors predispose to bacterial and yeast retention, which is accompanied by a typical smell. Oral infections
Oral ulceration
Fasting
Stress/anxiety
Menstrual cycle – at mid cycle and during menstruation, increased breath VSC were reported in women. Smoking – Smoking is linked with periodontal disease, which is the second most common cause of oral maloduor. Smoking also has many other negative effects on the mouth, from increased rates of dental decay to premalignant lesions and even oral cancer. Alcohol
Volatile foods  – e.g. onion, garlic, durian, cabbage, cauliflower and radish. Volatile foodstuffs may leave malodourous residues in the mouth, which are the subject to bacterial putrefaction and VSC release. However, volatile foodstuffs may also cause halitoisis via the blood borne halitosis mechanism. Medication – often medications can cause xerostomia (dry mouth) which results in increased microbial growth in the mouth. Nose and sinuses
In this occurrence, the air exiting the nostrils has a pungent odour that differs from the oral odour.