If you ever receive an email offering you a once-in-a-lifetime deal, like assisting a troubled Nigerian Prince in protecting millions of dollars from a rogue agency, it is most likely not legitimate.
Generally, a google search including a sentence from the email will turn up a great deal of information. It will, in most cases, tell you immediately if the email has been confirmed as a scam.
Check links. Many emails will ask you to login to www.yourbank.com, when in fact it is sending you to a completely different website that looks like yours. Be sure to always check your address-bar, to verify your location.