While there are various ways to determine if a $100 bill is fake, here are some security features of a legitimate bill that are easy to identify.
Place the bill in front of a light
When held in front of a flashlight, lamp or other light source, the $100 dollar bill displays several security features. Starting on the left, notice a mirror image of the 100 from the back of the bill is clearly visible, a blue line appears to the left of the newly visible 100, and a faint portrait of Ben Franklin fills the blank circle above the gold 100 on the bottom right side of the bill.
Tilt the bill
By bending the bill forward and backwards, see that the 100 and bells in the blue strip to the right of Ben Franklin's portrait move up and down. A bell also appears in the inkwell to the right of the blue strip, and the 100 in the bottom right hand corner of the bill changes color.
Feel the bill
On the left side of the bill on Ben Franklin's shoulder, feel raised printing when you move your fingers up and down the bill.