This depends on the type of email it is. If it falls into the "Too good to be true" category, then go to snopes.com and search for the subject of the email. Snopes is in the business of verifying or disspelling the validity of fabricated emails and other urban legends. You can also go to the search screen on this site and type in your Snopes search in the Snopes textbox and it will do a Snopes search for you.
If the email appears to be from a legitimate source but is asking you for sensitive information DO NOT GIVE IT. Either go directly to the site from the Internet (without linking to the site from the email) or call the company directly. Many times links in emails that appear to be valid will be taking you somewhere else entirely where you might unknowingly be giving your information away to the wrong party. Here is how you can tell. Look at this link:
You might be tempted to click it and give Chase your information because you trust the Chase name. Now hover over the link and you will be able to tell where you will actually be taken. Scary huh?
Another indication of fraud is when a link actually resolves to is an IP Address instead of a name. If you were to see 184.108.40.206 or something like this you can be sure that this is not a reputable company.
The bottom line here is that you should take your time, use common sense, validate the email if you can, and only provide sensitive information directly (by phone if possible and only if you make the call).
If you have emails you are unsure about, please send them to me before you send them out to everyone in your address book. I am always happy (as a self-proclaimed "white hat hacker") to assist in the reduction of spam and misinformation.