It depends what kind of addiction you are talking about. Some addictions, like an alcohol addiction or a crystal meth addiction, have unique characteristics like one's teeth falling out, or cirosis of the liver. However, in a broad aspect, addiction can include anti-socialism, lack of motivation/loss of goals or ambitions, extreme temper changes/mood swings, noticeable change in attitude and demeanor, and - most obviously - excessive indulgence into that which you are addicted to. If there are more questions you may have about addiction, check out the "Addiction Dictionary" I included in the "Related Links" Section. Hope This helped!