Sure. So simply asking who their auditor is and what the custody arrangements are will tell you a lot. Hedge Funds and other asset managers use a relatively small number of accountants to do their auditing. Madoff used a small family accountancy.

https://en.wikipedia.org/wiki/David_G._Friehling

This is not to excuse the SEC or let them off the hook. The SEC proved to be ultimately grossly incompetent at best, if not negligent or actually complicit. Investors should no more rely on the SEC to protect them than the hens should rely on the fox to protect the henhouse. This has been demonstrated repeatedly.

https://www.barrons.com/articles/when-chinese-stock-fraud-wa... https://www.wsj.com/articles/SB123577641445497313

Finally there is a statute of limitations when clawing back money that people received. I believe in Madoff's case it was two years. Additionally there were a number of lawsuits against a subset of investors that allegedly new about the scheme and profited from it.

https://www.reuters.com/article/us-madoff-trustee-opinion/ju...