Can't tell whether your iPhone is an original, 3G, 3GS or 4? Read this guide to find out how to recognise what version your iPhone is.
First thing to check for is the shape of the iPhone. If your iPhone has two cameras, one on the front and on the back and it is the same shape as the iPhone pictured below, then your iPhone is an iPhone 4.
iPhone 3G or 3GS
In order to tell whether your iPhone is a 3G or 3GS you need to look at the back of it. The iPhone 3GS has silver writing for both the Apple logo and the imprint down the bottom. The iPhone 3G only has silver writing for the Apple logo, the writing down the bottom IS NOT IN SILVER. This can be seen on the images below
Finally, if your iPhone is none of the above then it is the original iPhone, the iPhone 1. This iPhone is very distinct with its metal and plastic two colour back, shown below: