If you have reason to believe that an adult customer is buying tobacco for a minor, the responsible thing to do is refuse the sale.
Look for these signs that a third party sale may be taking place:
You notice people approach the adult and hand him/her money.
You overhear minors asking an adult for tobacco products.
An adult makes a multi-brand tobacco purchase.
If you suspect a third party sale, try using the following approach:
Ask the customer if they are buying tobacco products for a minor. Explain that sometimes kids hang around the store and ask adults to buy tobacco for them.
Explain that you could lose your job for making such a sale (in some states it is illegal for customers to attempt to purchase tobacco for minors).
It is important for your store to be seen as a responsible retailer. If you can reasonably determine that an adult customer is buying tobacco for a minor, the responsible thing to do is refuse the sale.