I agree with Sky, encourage your teen to go see a therapist of her own. She will be more likely to confide when you are not present, anyway. If her therapist discovers anything "reportable", she can take action, plus your daughter can get some therapy, which it sounds like she would benefit from.

Is there any way you can take your ex-husband to therapy with you and get some of this out on the table? Maybe in the safe environment of the therapy office, you could share your worries and fears. And maybe get some reassurance? Or maybe get more info that would confirm your fears? I don't know your situation well enough to know if that's at all realistic for you.

Take care.