How do you distinguish someone who is an alcoholic from someone who drinks but is not an alcoholic?

Not unless they hit me or my car while under the influence.

http://www.aa-uk.org.uk/publications/areyou.htm

To be diagnosed with alcoholism, you must meet criteria spelled out in the Diagnostic and Statistical Manual of Mental Disorders (DSM), published by the American Psychiatric Association.

Criteria for alcoholism to be diagnosed include a pattern of alcohol abuse leading to significant impairment or distress, as indicated by three or more of the following at any time during one 12-month period:

Tolerance, indicated by an increase in the amount of alcohol you need to feel intoxicated. As alcoholism progresses, the amount leading to intoxication can also decrease as a result of damage to your liver or central nervous system.

Withdrawal symptoms when you cut down or stop using alcohol. These signs and symptoms include tremors, insomnia, nausea and anxiety. You may drink more alcohol in order to avoid those symptoms.

Drinking more alcohol or drinking over a longer period of time than you intended.

Persistently having a desire to cut down on your alcohol intake or making unsuccessful attempts to do so.

Spending a great deal of time obtaining, using, or recovering from alcohol use.
Giving up important social, occupational or recreational activities.

Continuing to use alcohol even though you know it's causing physical and psychological problems.

http://www.mayoclinic.com/health/alcoholism/DS00340/DSECTION=tests-and-diagnosis

You can determine an alcoholic when their drinking gets out of control and it intefers with their relationships, job, etc.

http://www.livestrong.com/article/79494-factors-contribute-alcoholism/
The fine line between alcohol abuse and alcoholism, also called alcohol dependence, involves a physical dependence on alcohol. While alcohol abusers maintain the ability to set limits on their drinking, alcoholics do not. Common signs of alcoholism include physical dependence on alcohol, along with neglecting responsibilities, using alcohol in dangerous situations, continuing to drink even though it impacts relationships and legal problems related to drinking. Alcoholics feel compelled to drink and use alcohol to function on a regular basis.

Alcohol Abuse Factors
On-going abuse of alcohol may lead to alcoholism. Drinking more than 12 drinks per week for women and more than 15 drinks per week for men increases the risk of developing alcoholism, states the Mayo Clinic.
Stop Drinking Now A New Way To Help Alcoholics Stop Drinking In Just 8 Weeks! DrinkLessNow.com

Genetic Factors
Genetics may cause an increased tolerance for alcohol, risk of alcoholism and a craving for alcohol. A direct genetic link has not yet been discovered, says the Mental Health Channel. However, one study of identical twins showed the same level of alcohol dependence, even through they were raised in separate environments through adoption.

Gender Factors
Men are five times more likely than women to become alcoholics, states the Aurora Health Center. Women are more likely to become alcoholics later in life than men, and the dependency progresses more quickly in women than in men.

Psychiatric Factors
Alcohol abuse often impacts individuals diagnosed with psychiatric issues including schizophrenia, personality disorders, depression, anxiety disorders and PTSD (post-traumatic stress disorder). Also, people with other substance abuse issues such as illicit drug abuse or smoking are more prone to alcoholism.

Cultural Factors
Alcoholism seldom arises in cultures where alcohol is not part of the fabric of society, for example, Muslims and Mormons--members of the Church of Jesus Christ of Latter Day Saints. In Europe and the United States, however, where alcohol is considered socially acceptable, alcoholism rates are high. Alcoholism rates also relate to ease of access and peer pressure.

Psychological Factors
Alcoholism rates increase for people with certain psychological factors including feeling inadequate, high self-expectations, low frustration tolerance, high need for reassurance and a predisposition toward impulsivity and aggressiveness. Children with poor socialization, limited coping skills, conduct disorders and poor parent-child relationships are also at risk for alcoholism, suggests the Mental Health Channel.


Read more: http://www.livestrong.com/article/79494-factors-contribute-alcoholism/#ixzz1YOpL2F6o

Lot of good answers here.

When I was in HS, someone from AA gave a talk to my health class. In his opinion, the genetic predisposition (we weren't using that terminology back then) to drink uncontrollably is what made him an alcoholic, which is how he defined the term.