All WMDs leave a traceable signature of manufacture, even after usage. Didn't know that did ya? The big question is where and when- not if...
Yes and no.
An anthrax weapon, for example, using the Ames strain which was made in Iraq would show little differences from an another weapon using the Ames strain that was manufactured elsewhere.
The same would be true of fissile materials that were purchased from Pakistan or North Korea. The location of manufacture could probably be ascertained but not the country, or group, who had actually put it in a weapon.
Well, no. I didn't know that since to my knowledge that is a false statement (in the sense that the signature would be unique to Iraqi WMDs), but even to whatever extent it may be true, unless we have a signature to compare them against, any WMDs might as well be from Kiribati for all we would know...
We need to find verifiably Iraqi WMDs before we have any kind of signature of manufacture against which to compare WMDs found in, say, Syria. If we had verifiably Iraqi WMDs to compare against, we wouldn't be having this conversation..