There are some specific criteria for the diagnosis of ALS known as the El Escorial criteria . (They're named after a conference center in Spain, where they were developed in 1990.)
According to the El Escorial criteria, a diagnosis of ALS requires the following:
A thorough medical and family history and physical examination are the starting points of a neurologic work-up, which includes simple, in-office tests of muscle and nerve function.
If ALS is suspected, the next step is usually an electromyogram, or EMG. This test measures the signals that run between nerves and muscles and the electrical activity inside muscles to see if there’s a pattern consistent with ALS. If there is, more tests likely will be ordered.
Additional tests may include imaging of the spinal cord and brain, usually by MRI (magnetic resonance imaging), and sometimes a test of the fluid surrounding the spinal cord (spinal tap or lumbar puncture), which involves inserting a needle into the back between two lower vertebrae.
Blood tests to exclude disorders that mimic ALS are also performed. In some instances, a muscle biopsy, which involves taking a small sample of muscle under local anesthesia, is performed.
In some cases, genetic tests (performed on a blood sample) can confirm the diagnosis (see Causes/Inheritance ).
However, the diagnosis of ALS is often a “rule-out” procedure. This means ALS is diagnosed after all other possibilities have been ruled out by specific tests.
Among the conditions that resemble ALS are some forms of muscular dystrophy, the neurologic conditions known as spinal-bulbar muscular atrophy  and adult-onset spinal muscular atrophy , the nerve-to-muscle transmission disorder known as myasthenia gravis , and various causes of compression of the spinal cord or brainstem, such as tumors and malformations.
If your condition has been diagnosed as ALS outside a major medical center or without extensive testing, it may be worth getting a second opinion.