How can you identify a pyramid scheme?

A pyramid or Ponzi scheme uses money from new investors to pay "dividends" to the existing investors. If you have to recruit new "investors" to make money, it is a pyramid. You get in by paying someone upstream and are paid by the people you recruit, usually sending some of the money up to the person who recruited you. Legitimate multi-level marketing is similar, but different because you have to sell something but can make more money by recruiting people to work under you.