Change in Friends
This isn't always the easiest area to recognize, because it's a fact of life that friendships fade and new people come into our lives. But when normally social people stop hanging around with their usual group of friends with no explanation, it's important to take notice -- especially if there wasn't an event that precipitated the change. Or, maybe there's a new person in their life that just doesn't seem to be someone they would normally be friends with. This can be a telltale sign that something funky is going on, and it's particularly vital to watch out for this behavior in teens.