Notice the rectangular section on the Receiver, just above and to the right of where the Mag is inserted. That's how you can tell, these are called "lightening cuts", to shave a little weight off the receiver.
The Stamped versions all (generally) look like this one:
notice the dimple in the Receiver to help hold the mag in place (keep it tight within the receiver) and of course the rivets, which the Milled versions don't have.
According to the documentary last night, Milled AKs were about 2lbs heavier, but as John said, usually have smoother actions and contribute to more accuracy. It was a trade off, making the AKM lighter, but less accurate (not a whole lot I would imagine).
You may have already discovered that, among other differences, the stamped can be significantly less expensive.
One rationale seems to be: save the money and go stamped if you're getting an AK74 in the light recoiling 5.45, but fork the extra for a milled if you want an AK47 in the more [email protected] 7.62 :razz: :razz: :razz: (you can see where my preference lies).