That is the kind of question I would use to start a technical audit.
There will be code to review, systems to inspect and tests to check. But first, agree on what the business needs to understand. Otherwise, you can end up with a long report and still not know what to fix first.
Take a payment. Recording the charge is only part of the job. The customer also needs access to what they bought. If that step fails, someone needs to notice and sort it out.
Following that whole story gives the audit a clear starting point. It also brings the people who handle the problem into the discussion, including support and finance.

