How to know iPhone 4 or 4S Quickly

Between iPhone 4 and 4S look like the same, somewhat difficult to distinguish each other visually from the outside physically.

To know quickly which one  iPhone 4 and which one  iPhone 4S from outside physically, visually :

Look at iPhone from upside
See the figure below :

Easy way to know iPhone 4 or 4S
You can see the difference position of line/stripe of each iPhone.