The awesome Ben Heckendorm does it again! He's found a simple way to check if your 360 has the added heatsink. You look in the bottom vent! D'oh! If you can see through to the DVD drive, the new heatsink isn't there as it would block your view.
Here's the link to his site, with pictures, which is loading rather slowly.
http://benheck.com/08-02-2007/see-if-you-h...ty#comment-2904
And here's a quick screencap I posted on my flickr:
"Here's the text of what's on his page:
Or as I call it 'DIY Xbox 360 Endoscopy'.
The lens should “focus around” all the small holes. Be sure to use a flash so it lights up the inside.
As you can see my XBox 360 Core, purchased in June with a build date on the faceplate of 11/07 does not have the new heat sink since we can see straight through to the side of the DVD player. Darn. Good thing I kept the receipt.
So now everyone can check their new Elites, Premiums and Cores to see if and when the new heat sinks started showing up in units. You can find a manufacture date behind the not-voiding-to-remove faceplate, which of course is just the date of the faceplate but should give some indication."
Of course the manufacture date is also stamped on the back with the serial number and other info by the ethernet & A/V ports. An alternate method involves a bright flashlight & your imagination.