So, hopefully you know which model your iDevice is (iPhone 5S, iPhone 5C, iPad Air, etc). But maybe it's an older one you've had lying around, or maybe you got it from a friend or off the net and don't know the model. You can easily find out which device you have.
Check the back of your device, and read off the Model No:
Simply check your model number against the following table to see which device you're holding.