2G has a 6.5 Inch TFT Colour Screen 1/4 VGA - 580 * 240 resolution - 3G has a 7 Inch TFT Colour Screen VGA - 800 * 480 resolution
3G has dynamic Radio lists (it lists the radio stations its "seen" lately)
2G plays MP3 from CD only - 3G plays MP3's from CD, SDCard. Also plays DVDs on the screen (when staionary unless you hack it with VagCom)
Both have Connectivity for MP3 players via USB, iPod interface and Aux Audi (check you have both the USB and iPod Cables in the car)
2G has a max of 160 Directory entries in the telephone list - 3G has 2000. 3G does not need voice training to call (its pretty good at guessing the "name v spoken name" - 2G needed training with voice tags for the telephone directory
2G has 2D and Birds eye mapping for Sat nav. destination entered by spelling the word - 3G has 2D, birdseye, 3D terrain modelling and faster graphics, Destination can be entered by voice commands
As others said easy visual check is the joystick in the centre knob, other ones is the use of the words "Media" on the 3G and "CD/Aux" on the 2G (beside the Radio button)
3G centre console
2G centre console
Sepang blue

best colour
Lets see some pics when you get it
G
Ex RS6 C6 Driver, gone but not forgotten.. The best car I have ever owned