From http://urbanlegends.about.com/culture/urbanlegends/library/howto/hthoax.htm

Without researching the factual claims made in a forwarded e-mail there's no 100% sure way to tell if it's a hoax, but here you'll find common signs to watch for.

Difficulty Level: Average
Time Required: 15 minutes

Here's how:

1. Note whether the text was actually written by the person who sent it to you. If not, be skeptical.
2. Look for the telltale phrase, "Forward this to everyone you know."
3. Look for statements like "This is not a hoax" or "This is not an urban legend." They usually mean the opposite of what they say.
4. Look for overly emphatic language, the frequent use of UPPERCASE LETTERS and multiple exclamation points!!!!!!!
5. If the message seems geared more to persuade than to inform, be suspicious. Hoaxers are out to push emotional buttons.
6. If the message purports to give you extremely important information that you've never heard of before or seen elsewhere in legitimate sources, be suspicious.
7. Read carefully and think critically about what the message says, looking for logical inconsistencies, violations of common sense and obviously false claims.
8. Look for subtle or not-so-subtle jokes, indications that the author is pulling your leg.
9. Check for references to outside sources. Hoaxes will not typically name any, nor link to Websites with corroborating information.
10. Check to see if the message has been debunked by Websites that cover Internet hoaxes (see below).