How do you know when a person is an alcoholic?

Observe the person's behavior when they are unable or it is impractical to get a drink for about 24 hours. Alcoholics undergo profound personality changes when deprived of alcohol for prolonged periods. They also tend to shape their lives around drinking events.

Finally, if you believe a person that you know well to be an alcoholic, you are most likely correct. Our subconscious mind is a great observer.