Extreme physical changes are a big indicator of a problem. Maybe they're getting scraggly and smelly because they've stopped grooming or showering. This may not differentiate one teenage boy from the next, but it's something to keep an eye on. A noticeable weight loss or gain can be a symptom, and the same goes for a change in eating and sleeping habits. For example, crystal meth addicts can stay up for days, then crash and sleep for days. When they eat, they're likely to ravenously consume sugary foods.