Checking for:

• Sudden change in behavior
• Family isolation
• Irritability for no apparent reason
• Change in sleeping habits , food , hygiene and clothing
• Break school / professional income
• Changing the emotional state ( mood , euphoria , depression , insomnia , etc.)

However have the utmost care not to label your child , husband, wife, student or employee as they may present some symptoms mentioned above and not drug users . The best way to be certain is asking for professional help to experts.