Apple ipod

How I can know if the iPhone is 3G or 3G (S)?

I'm buying an iPhone used by anyone. How I can know if the iPhone is a 3G or 3G? I can pay for 3G, but I do not want a mistake and instead of buying a 3G.

See installed applications. If she is a compass 3GS. If not, is a 3G. Another suggestion helpful Watch the return envelope, if the text at the bottom of the phone is gray, is a 3G, it is a shiny, silver is a 3G.

iPhone 3GS: Tips, tricks, and features.