See below. The person takes the substance and cannot stop. Withdrawal symptoms - when body levels of that substance go below a certain level the patient has physical and mood-related symptoms. Addiction continues despite health problem awareness - the individual continues taking the substance regularly, even though they have developed illnesses linked to it.
You would know. Addiction is being presented with a choice and having a proclivity to choose a bad one to get the addictive substance. First sign is choosing the addictive substance over you. Methamphetamine, marijuana, cocaine and all behavioral addictions (gambling, etc.) do not have much in the way of physical withdrawal. You don't have to develop an illness from the addiction.