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It is hard to tell the difference between an iPhone 4 and an iPhone 4S as the hardware specs are almost identical. Almost.
This site has a good rundown of the minute differences, but if you want a quick "tell," check the top of the phone where the headphone jack gets plugged in. On the iPhone 4, there's a black line next to that hole connecting the front and back of the device. There's no such line on the iPhone 4S. You can see each version in the fourth picture down from the top of the article.