If you are a male Wal-Mart associate, it's pretty straightforward.
When a frumpy 40-something lady with no makeup and a massive headache who has just spent the last two hours on breakfast-making, lunch-preparing, hair-brushing, scripture-reading, backpack-packing, note-signing, shoe-finding, carpooling duties happens to come through your check-out line with a package of Sharpies in her cart, all you have to do is look down at her over the rim of your glasses and ask,
"Are you over 18, young lady? You know you have to be over 18 to buy Sharpies, don't you?"
It's as simple as that.