When students have to identify coins as they hold them, they get it pretty quickly.
However, on a test...they can easily mix them up. For one thing, the penny is often shown as black and white just like the others. You can show these on a smartboard or simple print them off for students to color.
Use the coin posters to show the differences on each coin. Knowing which president is featured on each coin is a fun bit of trivia as well.