> Does it work?
There's no reason why it shouldn't.
> I mean, it's much easier for a tax inspector to simply go to the store, when it's busy, and wait for the slip up (when the clerk accepts money but does not use the register).
Why would a lowly clerk break the law for his multi-millionaire employer? And why would a customer do business with a company that didn't provide receipts?
That's not how it works. The fraud happens further up the food chain. However, if the taxman knows exactly how much money came in from customers by monitoring the cash register, he knows how much to expect in taxes from that business.
> But in my opinion this was rare even before.
Perhaps in your country. A few years ago whistleblower protections and a reward system were enacted here. So anyone who dobs in a company that's been evading taxes gets a percentage (IIRC 10%) of the unpaid taxes subsequently recovered. Since then there's been a string of highly-publicised cases of the taxman coming down on tax evaders. They recover taxes and squeeze more out of them in penalties and fines, and the whistleblower (probably some low-paid bean counter in the company) get's a huge lump of cash for his troubles.
Anyway, as a result of these events, now the taxman monitors the tax registers of large companies that accept cash. Certain categories of commerce and small businesses are exempt.