Engadget has an interesting (if gratuitous) disassembly of a new Jasper-era Xbox 360. But the most important thing here, of course, is the info on how you can ensure you’re getting a Jasper unit when you buy a new Xbox 360:
The Xbox 360 box has a hole at the top of it where the bar code of the console is visible. Clerks usually scan this when you buy one. What you need to do it look at the lower left corner of this box. If you peer closely you can make out at least half of the voltage/amperage rating of the unit. The # of amps used by the 12 volt rail is the key to determining which console it is. The photos below show this in detail.
14.2 amps equals a Falcon unit (65nm CPU, 90nm GPU). Please keep in mind there's nothing wrong with having one of these, I play mine a lot and it's fine. You might not be able to see the whole voltage listing just through the peep-hole, but you should at least be able to see if a 14.2A or 12.1A is just to the left of the "5V".
12.1 amps equals a Jasper unit (65nm CPU, 65nm GPU). This voltage rating is also listed above the power input, but of course you can't see that when it's in the box.
There has been a lot of discussion about lot #'s, power supply ratings, and things of that nature being used to find Jaspers. I have found those to be unreliable.