If a teen suddenly withdraws from family life, or if they've become more secretive, defensive, or aggressive when you try to talk about it, it's a good indication that your teen is sexually active. On the flip side, some teens (particularly girls) may start dressing inappropriately and clearly crossing the boundaries of acceptable behavior in public. The real danger is that a sexually active teen is more likely to start drinking, smoking, and using drugs. Any sign of these related activities should alert you to the possibility that your teen is sexually active.