How to tell an iphone 3g or 3gs apart?

My sisters friend is selling me an iphone, but she can't remember whether it's an iphone 3g or an iphone 3gs because she's had it over four years I believe. How can you tell them apart?

Iphone 3g can't have a wallpaper on homescreen and the ios only goes to 4.2.1. Iphone 3gs could have a wallpaper on your homescreen and 3gs could go be upgraded to ios 5 and 6 i believe

Look on the back of the phone, in the small print.
If it says "A1303" or "A1325" somewhere, it is definitely an iPhone 3GS,
If it says "A1324" or "A1241" somewhere, it is definitely an iPhone 3G.

It will say something like, "Designed by Apple in California, assembled in China, Model" and whatever comes after the word 'model' is what you're looking for.

If it doesn't say or you can't find it. Go to Settings > 'General' > 'About' and type the Serial Number here: https://selfsolve.apple.com/...ntyDynamic. Do and it will say the type of phone it is along with other information.

Hope I helped.
Why am I getting thumbs-down for this? Bit unfair.

Well physically they can't be told apart, aside from 1 small difference. The writing on the back of the iPhone 3GS is in chrome vs white on the 3G.

Flip over the phone and look at the writing, if it's in chrome it's a 3GS and if it's in gray it's a 3G.