3D Security Ribbon
Color-Shifting Ink
Portrait Watermark
Security Thread
Raised Printing
Feel the texture of the paper.
Notice the thinness of the bill. 
Compare the bill with another of the same denomination and series.
Inspect the printing quality.Look at the borders.
Observe the portrait.
Examine the serial numbers.
Hold the bill up to the light.
Use an ultraviolet (black) light to look at security threads.
Check for watermarks.
Tilt the bill to examine the color-shifting ink.
Examine the micro-printing.
Several of the provisions in the federal counterfeiting law provide for a fine of up to $250,000 and a prison sentence of up to twenty years for the counterfeiting of U.S. obligations and securities.
Questions for pictures #1-4
What is happening in the picture?
The money is being exchanged.
What are the people doing in the pictures?
The customer is exchanging money with the cashier.
Do the people seem happy?
Since they are bank tellers, they look depressed.
What can you tell about the people/item in the picture?
It's a machine that counts money.
What is the consistent item in the pictures?
The fact that it looks the same but are completely different bills.
Would the people's moods in the picture change if the currency being exchanged was counterfeit?
Probably, since counterfeit bills have sort of a different expression that an authentic one.
What do you think the issue is with counterfeit money?
You can't spend it unless you get caught.
Would you like to receive a counterfeit bill?
The obvious answer is no. But, as most people handle their money, when it's given to us, we don't check on whether or not it's authentic.
What would happen if you received a counterfeit bill?
If I knew it was a counterfeit bill, I would probably report it.
Question for pictures #5-6
Can you tell which bill is counterfeit?
Both the top bills are counterfeit due to the awkward change of the facial expression. The raised printing on the bottom show that it's authentic.