You can voice your concern to the Mother's prescribing physician, if you know who that is. If the physician chooses to pursue it, he can request drug testing be performed which will assess the amount of drug in her body and compare that to the ordered dosage and quantity. This will determine if she is taking the medication as prescribed. The physician must maintain patient confidentiality and will not disclose this information with you.
You may also voice your concern to the court and if the court sees to do so, may order mandatory drug testing as well.