Well no. If a customer pays with cash, a cashier can take payment for something without actually ringing it up. The sale never enters the computer system. Even if you audited the shops inventory, you'll nearly always discover some products are missing, but that doesn't prove employees have been stealing; shrinkage has a variety of causes (shoplifting, damage, was never actually delivered, etc.)

Regardless of theory, pay attention to where cameras in corner shops are pointed and I think you'll see what I've often seen: the cameras are primarily positioned to watch employees.