As above, just got given an iPhone with a broken screen.

Basically on the back it says 16gb, and it's a 3 of some kind.

Can I tell whether it's a 3g or 3gs with the numbers on the back?

Cheers

https://selfsolve.apple.com/GetWarranty.do

Head there, stick the serial number of it in, and it will tell you what model it is.

If you don't know the serial number, or how to find it, head here (if it is an iPhone 3G or later, it'll be on the SIM tray)

http://support.apple.com/kb/HT1267

You can indeed, and here's how:

http://support.apple.com/kb/HT3939