Every one I've been to has a POS computer/register where they enter every sale, no matter how it's paid. Enter the employee ID of the person that did the cut, the service, cash tendered, money drawer opens, provide change if applicable, offer receipt. Same as any other legit business.
There's a grey market in "zappers", which are POS devices that keep two sets of books automatically. Lots of small retail businesses rely on this type of tax fraud to stay in business.
