Whats stores computers don't ask for age on R and M rated games and movies?

I know that Walmart's computer tell the cashier to check the age of the customer as soon as an R or M rated game or movie is scanned. So I would ume at walmart that if you wanted R or M you gotta do the self checkout. And at bestbuy I know that they don't for unrated movies as I bought Harold and Kumar there. Anyone know what stores DO and DON'T id?