1. The offer is too good to be true.
In the real world, your money won't grow 50% higher in a day unless you put it in gambling, which is also a 50% chance of losing money. Example: In cloud mining, like hashocean, which is a ponzi, uses words like "return of investment in 3 months" and you get a 'fixed' amount of money per day.
I know that it seems a bit blurry if you are still not aware but if you look at how the system works you will know the points that makes it a ponzi. You would have to determine how the company or entity earns its money or how it profits. Why? Because every business plan's goal is about profit and if you see that the company's profit is too low to cover their expenses then you should bail out.
2. Referrals
Yes, why? Imagine you have a company and you grant each one of them 10% each referral. That is a big loss for your company, unless you have a more sinister plan in mind.
3. Membership fees
If you are trying to employ a person, would you make him pay before you employ him? I can understand if the payment is for the products but membership fees are indications that they are operating on 'circulating the money scheme'.
4. You fond out about it because of a facebook post or some random spam.
The reward system on getting more members will more likely be found as a spam on social networks or emails so watch out on facebook, steemit, reddit, or bitcointalk posts. The admins want to get as many investors as they can to increase the money in circulation so that in the end they can get away with the money. You must be critical on the things you read. If its too good to be true, then you are right so don't bite the bait.
-This post is not absolute and maybe false in other circumstances but it is based on real experience ;)
Great guide for newbies to check if any new upcoming coins is a ponzi scheme or not.