Smoking marijuana will cause your child extreme hunger to the point of eating anything in the refrigerator.
Smoking marijuana will cause your child to feel uttermost sleepiness even after a full night's rest.
Half-closed eyes could be a sign of marijuana use. Your child might try to disguise that by using eye drops.
Smoking marijuana might cause dry mouth to the point where your child will be drinking almost continuously.
Smoking marijuana will cause your child to laugh at anything uncontrollably.
Smoking marijuana will make your child confused, easily distracted, forgetful and unable to focus.
Should you notice any or all of the above signs, talk to your child immediately. Ask questions and be willing to listen, but do not resort to emoting and accusing, as that will only create a greater communication barrier between you and your child.