How do you know if your Iphone is a 3g or 3gs if you want to upgrade?

If you try to update beyond iOS 4.2.1 and it fails that is a pretty good indication you have 3g and not 3gs.

Trying to upgrade will not hurt it. You will just get an error message say it can't be upgrade that is all.

Allan

Connect it to iTunes on a computer and check for updates; if it shows iOS 4.2.1 or says that iOS 4.2.1 is the current version, it's an iPhone 3G. Alternatively, click here and check its specifications against those posted.

(80092)

Check you model no.

http://support.apple.com/kb/ht3939

You can find the model number on the iPhone's back case.