The email or requestor asks for bank account information, credit card numbers, driver's license numbers, passport numbers, your mother's maiden name or other personal information. (Automatic Red Flag!)
The prize promoters ask for a fee (for administration, "processing", taxes, etc.) to be paid in advance. A legitimate lottery simply deducts that from the winnings!
Reply addresses to these scams are usually either to a Yahoo.com or gmail.com or any other public email provider, scammers like to use these free services. Remember if some huge financial institution or legal firm wants you to contact them, they would have a private domain with their own email address.
Most of letters received are written by people who have a poor command of the English language with errors in punctuation, grammar and over use of ellipses (periods, asterisks, dashes, etc.)
When selling an item online, someone offers you more than your asking price, usually to cover the so called shipping costs and they want the item shipped to a different location or another country.
The caller or emailer is a victim and is in another country and now needs your help to return to the United States. Often times requesting you to send funds to them via Western Union.
The confidential nature of the transaction is emphasized.