How to tell if I have an iPhone 3GS or iphone 3G?

My friend sold me an iphone for 40 dollars it looks like the iphone 3G or 3GS but i don't know which one it is how can i tell the back of it is white

3GS is 320W,can take video ,3G is 200W,can't take video ,Setting about -MC is 3GS。MB is 3G

Look at the back the iPhone text. If the letter are grey then its a 3g. If its shiny like the apple logo then its a 3gs.
You can also tell by camera app. Look at the bottom right and if there's a potion to switch to video then its a 3gs if not then its 3g.

The model number would be a good start. Also checking to see what is the latest version of iOS for it.

3G - 4.2.1, 3GS - 6.1.3

This could help as well
http://support.apple.com/.../kb/ht3939

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