1) What types of chemical weapons were they? Sarin? VX? Exactly what weapons were allegedly smuggled into Syria?
2) Which countries have or possess the capability to produce these weapons? If they are only available in certain locations, then it would almost certainly have come from one of these locations.
3) Where did these weapons come from? The route to Syria is important for understanding who would have shipped the weapons there. If a certain route is commonly used by a certain group, then there is a good chance that there will be some connections.
4) Do 1) and 3) match up? If the weapons came from somewhere which does not possess the capabilities to produce them, there clearly is a problem with the accuracy of this information.
5) How long can the weapon last in storage (both properly stored and improperly). Iraq whose chemical weapons stockpile has not been secured throughout has sarin but according to this CIA document, the weapons have shelf-life problems and had degraded quickly
6) Is it possible for this weapon to be properly stored in Syria by rebels? Are there any special procedures that must be taken to assure that the chemical agents do not degrade?
7) Which groups received this weapon as it arrived into Syria? Are these groups capable of staging a large-scale attack over a several kilometer sized area?
8) Who are these groups aligned with? Are the others they work with more capable of making an attack of the scale of Ghouta?
9) Where are these groups located? Where do they operate from? Syria is insecure in many areas and certain factions have control of different areas. If the exact location of this smuggling is identified, then those responsible may be identified. If these groups are incapable of operating in Damascus, (due to lack of reliable transit and chemical weapons handling or whatever other reason), then the Ghouta attack almost certainly was NOT perpetrated by these groups.
Until Iran answers these questions, I believe there is no reason to question the American/English/French/German intelligence claims that Assad was almost certainly the one responsible for the Ghouta attack. It is possible that these chemical weapons allegedly brought into Syria were used at another location, but the length of time since this alleged incident and the timing of the revelation of this information is highly suspicious. It is clear that Iran is attempting to imply that the Ghouta Massacre was perpetrated by these smuggled weapons. Iran has made a serious accusation, but has provided so little evidence so that I find myself doubting their claims.