Fake checks usually are sent in conjunction with "send me cash back" requests, that's one signal. However, if everything seems normal, it's still possible you have been sent a fake check...

1. Check for watermarks. That's usually missing in forgeries.
2. Read the check carefully (usually they describe security features such as "the date line is actually the word REAL in very small letters" or such).
3. Call the bank (or company) listed as the payee, and verify the check number and amount with them directly.
4. Take it to the cops and have them look at it.
5. If it passes those tests, but you're still suspicious, you can deposit the check and not touch the funds until they are "honored" -- which is different from "cleared". The following quote is from http://www.fraudaid.com/check_liability.htm#Clear
   
   Quote:

   "Available funds" does not mean the money in your account belongs to you, even if a hold has been taken off the draft. When you deposit a check into your bank account, your bank advances you the money for that check to keep the wheels of commerce moving.... This credit to your account is called a "provisional loan" and is actually a no-signature loan from your bank to you. It does not mean that your bank has been credited by the account holder bank. The only time the money in your account really belongs to you is when the check or money order has been HONORED, meaning your bank has been credited (paid) by the account holder bank. ALWAYS ask your bank if the draft has been honored and DO NOT TOUCH THE MONEY until it has. When in doubt about a draft, tell your bank to send the draft for COLLECTION. When you send a draft for Collection, it means that your bank or credit union will not put any money in your account until the draft has been paid, i.e. your bank has received the credit from the account holder bank. Your bank or credit union will charge you a fee and give you an approximate time before the draft is honored. This time period may be as long as 6 to 8 weeks depending on the location of the account holder bank.

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If it's too good to be true, it isn't true.