Courtesy of Engadget, Ben Heck has recently dissected a Microsoft XBox 360 Jasper console and says that checking the amperage rating near the barcode (you can see through the hole in the packaging) is the most sure-fire way to find a Jasper to date.
In summary, if it says 14.2 amps it's a Falcon unit (65nm CPU, 90nm GPU), 12.1 amps makes it is a Jasper unit (65nm CPU, 65nm GPU).
Those interested can check out the full article (linked above) for additional details and complete disassembly documentation with step-by-step pictures.
Below is a picture (click to enlarge) of the XBox 360 packaging hole, with both the Falcon and Jasper differentiation illustrated. More PlayStation 3 News...