I recently got a lesson in how to spot counterfeit money. An artist friend of mine handles money in her day job and gave me some tips on what to look for. For $10 and larger, hold the bill up to the light with the face towards you and look for an identical face watermark on the right side. There should also be a security thread running top to bottom towards the left side.For when I don't have enough light, or I don't want to be so obvious, she also gave me a spiffy pen that you make a small mark on the bill and it will change color if it is not the real deal.
For more detailed info you can also check out this website: http://www.wisebread.com/how-to-spot-counterfeit-money