Look at the eyes of the portrait. On the counterfeit bill the iris looks like a black dot. If you look closely at a real bill, the eyes are made of circles which get closer and closer together, however never touch. Look at the background printing. Look at the flow of colours.
Real currency has a true rainbow effect flow of colours which is very natural.
Many of the counterfeit bills show a definite change from one colour to the next. Look for planchettes. These are the green dots you see in the notes. When real currency is produced, these planchettes are added during the paper making process, and as a result, each planchette is visible from both sides of the bill. These planchettes can also be removed from the bill.