• They ask you to transfer a fortune from their country to your bank account for safekeeping. But if they really had a fortune they wouldn't need to ask a stranger for help. They could easily afford the best lawyers money can buy to help them with this task...if they were legitimate.
• They say they got the money illegally and that's why they can't go through normal channels. If they admit to being crooks, why would you trust them?
• They pretend to be businesspeople or government officials and ask you to invest in a partnership. Real companies and government agencies don't contact strangers in other countries to offer business propositions.
• They send you a counterfeit check or money order as an "advance" on the millions you'll receive and ask you to send cash back for legal services, bonding, or other expenses. That is not how any legitimate deal works.