How do you know when an online job is a scam ?

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I have been surfing the internet day and night looking for something that works and up to know I haven't found anything. I have found that a lot of these online job require money up front. How can I pay for something that I am not sure of the results? The advertisement and presentation are great but, its all about risk and majority of the times you lose a lot of money. So how do you really know when its a scam ?