There is no definite method to ascertain the takings or profits of an existing business.
A purchaser can ask to look at the accounts, owner's tax returns and references from suppliers, creditors, the bank and accountant.
If a company owns the business, basic information regarding the company and its owners and directors can be obtained from the Australian Securities and Investments Commission. Before buying a business, it is best to spend a period of time running the business with the owner to gain familiarity with its operations.