There were some subtle outside changes even between version of the same model years. One way to tell is that the little LEDs for the computer and thumbs up/down are in the bottom corners on the newer ones where on the older ones they are both in the upper right area. The other (and probably the best way to tell) is when they turn on, the older models when turned on will show at the bottom from left to right: temp, storage, battery. The newer models the very bottom will rotate/scroll the display of the temp, storage, battery.