The financial system we have relies on a system of checks and balances intended to prevent manipulation / fraud from occurring.
The primary way this is done, is through the requirement that public stock exchanges have, for companies to obtain a 3rd party audit of their financial results. If you open the financial report of any public company, at the front page there will be a statement from an accounting firm stating that to the best of their ability to audit, the numbers are not 'materially misrepresented'. This means that any error (intentional or not) should in theory be negligible to the overall results being reported.
However there are a few flaws with this system: audit firms intend to 'consider, but not detect' fraud. This means it is in the back of their mind, but primarily they are looking for accidental misstatement or non-hidden errors in accounting policy. The truth is that fraud through collusion is incredibly hard / expensive to detect. So a firm will observe whether internal controls are faulty enough to allow fraud to be directed by a single individual, but if multiple individuals collude, that is very hard to detect.
This means that sometimes fraud does occur in the reporting of financial results, leading to a significant misstatement of earnings. The most famous example these days would be Enron. To be simplistic, they were recording revenue to be earned from utility companies from infrastructure that had not been built yet, with the underlying 'justification' that their history of growth projected that the revenue would soon occur. The failure of the accounting firm, Arthur Andersen, was so great that it no longer exists.
For a company to not be fraudulent, it requires a competent auditor, but more than that it requires an internal culture of honesty and a strong presence of a board of directors with a willingness to listen to their internal audit committee (which is kind of like 'internal affairs' in that it works for the company but doesn't report to the CEO, it reports directly to the board). This is no guarantee that fraud doesn't occur, but it's the system we have in place currently.
Various other measures have existed from time to time to increase the ability to rely on financial results as reported, with varying successes. The risk of misstatement is always there.