The Assoc. of Certified Fraud Examiners has a useful list of red flags of Ponzi schemes.
Sounds too good to be true. If an investment sounds too good to be true, it probably is. This old saying is perhaps the easiest way to spot a Ponzi scam.
And some of the perhaps less obvious red flags.
History of consistent returns
High-pressure sales tactics
Pressure to reinvest
Complex trading strategies
Read the whole thing. It might save your financial life.