Apple iPod Touch

First of all, make sure it has the Apple logo on the back of the iPod. Make sure is is easily visible from all angles and lighting. The fake ones usually aren't.

Next, make sure the screen quality is high. It should be clear and not pixelated.

Next, make sure it has all the originally installed apps on it and make sure they're spelled right. It should have:

FaceTime
Calendar
Photos
Camera
Videos
Maps
Weather
Passbook
Notes
Reminders
Clock
Stocks
Newsstand
iTunes
App Store
Game Center
Settings

Apps on the iPod Dock:
Messages
Mail
Safari
Music

Next, Connect to iTunes. If you connect it to iTunes and it doesn't show anything or you're receiving an error, it means iTunes does not recognize your iPod. It's a fake.

On the iPod, go to "Settings," then choose "General," and then click on "About." This will give you the serial number. You can then access the Apple website, where you can enter your iPod serial number to see if it is a real iPod.