Use Escrow if Possible?
Of course, not all payments can be escrowed. Bitcoin exchange deposits are a good example.
Mt. Gox is the most infamous case of a fraudulent exchange, in which 650,000 bitcoins were lost. There were many warning signs before Gox failed and its worthwhile to research the entire saga to learn what they were.
The more you learn about previous scams, the easier it becomes to recognize the signs.
Unless youre an active trader, a good rule of thumb is to never leave your bitcoins or fiat money on an exchange. Bitcoins outside your personal wallet are bitcoins you dont own.