As others mentioned, unexplainable business models or money coming out of nowhere are major red flags. A few others to look out for if you are applying for a job:
1. Few or no internal controls. A medium-size or larger company not having a CFO is particularly questionable.
2. People in high positions, including the CEO, lack qualifications or experience. Often the CEO will hire inexperienced or incompetent people for high-level positions because they a) won't figure out what is going on or b) can figure it out, but are too grateful for the position to say anything. Or maybe the CEO never finished college in a field where everyone has a PhD (think Elizabeth Holmes).
3. History of experienced / qualified people joining the company and leaving soon after
4. Seemingly unexplained success.
5. Extravagant spending and inappropriate workplace behavior.
6. Cult of loyalty to the CEO and/or a "circle" around this person, including close interpersonal relationships.