Your child becomes more secretive, and it seems like more than a desire for greater privacy
Your teen has regular, sudden outbursts of anger that are clearly unreasonable and disproportionate to whatever has caused the anger
Your teen regularly misses curfew, does not show up when expected, and lies about his or her whereabouts (is not where you had expected them to be if you check up on them)
Your teenager has suddenly changed his or her peer group and has not made an effort to let you meet these new friends. The new group has led to a distinct change in appearance (clothing, jewellry) and change in attitude (more sullen, defiant, hostile).
Your adolescent has stolen money from your purse on regular occasions.
Your adolescent has extreme mood swings, from depression to elation, and seems to sleep a lot more than usual at times.
Your child's grades have suddenly dropped and the child has lost interest in the usual activities.