The first is that Saddam was reported to be wearing glasses (sunglasses?) which is unusual. It's good for concealing the wrong eye color in case this is actually a double.
Second is that Saddam alledgedly took care to mention the date, 20 March 2003. That's not very conclusive. It wouldn't be that hard to pre-record a number of messages for various dates in the future. That's why visual images of newspapers are far more determinative - there is a host of other information that are much harder to predict than the date.