Bell’s palsy is a clinical diagnosis that can be established based on the patient's history and physical exam findings. It essentially represents a diagnosis of exclusion when all other possibilities are eliminated.

The timing of symptoms is extremely important, as Bell’s palsy typically has a sudden onset and progression when compared to other causes of facial palsy such as tumors, which cause a gradual progression of muscle weakness. Although this condition is the most common cause of acute facial paralysis, similar presentation of other conditions can have detrimental effects in the case of a misdiagnosis.

Since there are no readily available laboratory tests or imaging methods that can verify the diagnosis of Bell's palsy, the key is to differentiate this condition from other possible etiologies. Laboratory workup which includes CBC (complete blood count), erythrocyte sedimentation rate measurement, thyroid hormones analysis, HIV (human immunodeficiency virus) screening, serum glucose level, blood urea nitrogen, liver enzymes and cerebrospinal fluid analysis can help identify or exclude other disorders. If the paralysis does not improve or becomes even worse, imaging studies like magnetic resonance imaging (MRI) may also help rule out a tumor, especially if the patient has a palpable parotid mass.

Herpes zoster infections that involve the facial nerve and Lyme disease are sometimes difficult to exclude in the differential diagnosis. In herpes zoster infection there are usually small blisters and vesicles present in the external ear canal, accompanied by hearing difficulties. Serum titers for herpes simplex virus may be obtained, although this test may not prove helpful due to the ubiquity of this virus. Lyme disease can also produce facial palsy, and may be easily diagnosed by looking for Lyme-specific antibody titer in the blood and finding a circular expanding rash on a physical exam. In endemic areas, Lyme disease often represents the most common cause of facial palsy.

Electroneurography, which uses electricity for stimulating facial muscles on both sides of the face, may be used to provide prognostic information in cases of complete facial paralysis. Degree of nerve degeneration can be successfully quantified by mutual comparison of the responses on both sides of the face. The hearing threshold is usually not affected by Bell palsy, thus audiography and auditory evoked potentials should be pursued if hearing loss is suspected. Computer-based analysis of eyelid motion (also known as blepharokymographic analysis) may prove helpful in establishing diagnosis, predicting prognosis and evaluating therapy response.

Two different diagnostic scales are used to grade the severity of Bell's palsy. Those are House-Brackmann Facial Nerve Grading System and the Sunnybrook Facial Grading System. House-Brackmann system categorizes the condition into six different categories, with first category representing normal facial function and sixth category a state of complete paralysis. It can also help clinicians monitor disease progression and evaluate recovery of the patient. The Sunnybrook system is a recommended method for subjective assessment of the mimic muscles and facial symmetry using a scale from 0 to 100. Although both scales are useful, several aspects of Bell's palsy such as tearing and facial discomfort may not be adequately assessed.