How will SARS know what's petty cash and what's income?

I get petty cash from my company deposited in my account throughout the month and year, how will SARS know it's petty cash and not an income?

SARS doesn't review your bank account. It is up to you to declare the correct income when you submit your tax return. If you are required to submit bank statements to them for some reason, then you just need to highlight the petty cash entries and explain what it relates to (ie reimbursement for expenses and not income)