1) Get rich quickly. If there are fantastic claims of getting rich in a short amount of time, than it's definitely a scam. Although this can be done in some extreme cases, by gambling or playing the stock market there is usually extreme risk involved. So if someone approaches you saying that they can turn you into a millionaire tomorrow, run away!
2) No work involved. If you are not required to lift a finger and somehow cheques are magically appearing in your mail box. Than I'd like you to refer me to your mailman. Seriously though, nothing of value on this planet is created without some work. Only when value is being provided than money is exchanged. If someone is preaching about making a lot of money without a lot of work than there is something seriously wrong with this situation.
3) You pay first. Finally, you know it's a scam if you can't start making money until you have to pay someone. What's happening here is that you're being bamboozled out of your savings. A lot of times con artists will ask you to first send in a cheque before you get the information necessary to start making "millions". If anybody is making such claims your "spidey sense" should start tingling. If possible, end the conversation immediately and get the hell away.