1) Determine that claimed profits are in fact profits from investments.
2) Realistically assess the risk to investors.
3) Assure investors that there's a mechanism in place so that if they do take losses, they'll be able to reasonably assure themselves that those losses were due to legitimate investment losses and not just him deciding to keep people's money.
4) Assure investors that there's a mechanism in place such that if there are reductions in the total worth of all investments, losses will be taken proportionately by all investors instead of processing full payouts to those who happen to withdraw first and leaving essentially nothing for those who don't withdraw at the right moment.
It's not a matter of whether it's a Ponzi or not really. There are lots of scams that aren't Ponzis. The idea is to show that it's a legitimate loan or investment. If there was something that was otherwise a legitimate investment but couldn't do at least two of these four things, rational people would not invest in it.