Change in Behavior
As people's addictions get bigger, their lives get smaller, and the things that they value the most begin to fall away. Depression sets in, and suddenly they stop doing activities or hobbies that they once loved, or maybe their mood has become erratic. An example of a change in behavior that's worth noting would be when a person who values health and fitness abruptly stops exercising or eating well. Teens are notorious for wanting privacy, but when their efforts to keep you out of their space become extreme, you might want to examine more closely.