If you receive an email from a Nigerian prince/princess letting you know that they are in trouble but they will give you several million but only if you give them your bank details - this can be a good indication.
Another famous email scam is a foreign lottery/competition letting you know that you have won millions. This would be fantastic but if you have never bought a ticket, how likely is this to be true?
There are variations on these themes but they usually involve offering a large amount of money in unlikely circumstances.