Camera Differences
The iPhone 6s and iPhone 6s Plus have much more advanced cameras than their predecessors. The front cameras have a "FaceTime HD" camera with a 5 megapixel sensor compared to a 1.2 megapixel camera on the iPhone 6 and iPhone 6 Plus.
Identification Differences
It is easy to differentiate between the iPhone 6, iPhone 6 Plus, iPhone 6s, and iPhone 6s Plus when they're side-by-side based on dimensions and the presence or absence of an "S" on the back, but identifying the specific device requires more precise data.
