As per the OP’s questions, SARS does not technically check your bank account. They request bank statements from your bank, reconcile, and see if it matches up to your income declared on your tax returns.
In terms of physical cash, banks report amount of 25K and over, to the Financial Intelligence Centre. The FIC may report any suspicious activity to SARS or other law enforcement agencies.
Most taxpayers are never ‘audited’. They are selected for verification. If you declare x as your income, and y as your expenses, then you need to provide proof of x and y. Usually some system flags taxpayers if there is a discrepancy of their tax profile.
SARS usually only looks back at the past 5 years when auditing/verifying, however, if they suspect fraud, they can go much further. That said, the tax act mentions tax payers should keep their records for 5 years.
You pay tax when you start earning R87,300 a year (2022 tax year). Even then, your annual tax is R18.
As tman and chrisc mentioned, SARS has no time to worry about your pennies. There would focus resources on bigger fish (i.e. Dave King). That said, if SARS wants to pick low hanging fruit, tomorrow they can look into rental income. SARS uses Third party returns (i.e. rental agencies, banks, attorneys etc.). Rental agencies are obliged to report rental income information to SARS. So good luck to those that think they are “beating” SARS. If a billionaire couldn’t beat SARS, what are your chances?
If you need advice, speak to a tax accountant. if you cannot afford one, Tax Tim appears to be the cheapest out there, and charges R239 per basic tax return. If you still cannot afford this, go to a SARS branch.