If you've been using email for long, you've probably had someone send you a warning about a dangerous new virus.  99.99999% of these warnings are hoaxes, but how do you know for sure?  Here are some sure-fire ways to recognize a hoax:

1) No specific date - the alert says the virus was discovered "yesterday" or is coming in the "next few days", but doesn't give a specific date such as "Dec 13, 2008". 

2) Tells you to send this alert to all your contacts.  This is how the hoax-writer gets you to spread his hoax.  No legitimate virus alert would ask you to do this.

3) Claims this was announced by Microsoft, Symantec, McAfee, CNN, etc.  These organizations do not send virus alerts by email, and if you check their websites you'll find no mention of the supposed virus.

4) Claims you can get the virus just by opening an email - to get a real virus you'd normally need to open an email attachment or download a file.