Here's an old report, from April of this year.
http://worldnews.nbcnews.com/_news/2013/04/28/17959388-fighting-reported-near-suspected-chemical-weapons-site-in-syria?lite
Barzeh is pretty much due N of downtown Damascus. A N-NW of the center of the city, past the suburbs (sort of defining where the suburbs stop) is Mt. Qasioun, which has hills leading E-NE from the highest elevation. They're sort of a northern limit to Damascus at present.
hills
Mt. Q
Barzeh
(Damascus proper)
(Let's see how DU reformats that)
al-Ghouta is built up suburban areas adjacent to Barzeh. It's not a single spot, it's a strip.
If there's a single most likely place to have chemical weapons used, it's there. If there's a single most likely place to try to do a false-flag operation so everybody suspects Assad used chemical weapons, it's there. If there's a single most likely place for a chemical weapons brigade to be stationed, it's there. If there's a single most likely place for some local commander to panic and use chemical weapons, it's there. If there's a single most likely place for some intermediate commander to order them to use whatever they have and for "whatever they have" to be chemical weapons, it's there.
All within one to two miles.
Depending which precursor chemicals you start with, it can be a little complicated to make sarin or absurdly simple. If I could get one of the two immediate precursors I could make it in high school. I have one of the two essential precursors in my bathroom, isopropyl alcohol. I can get one of the less immediate precursors from my drinking water, NaF. A good university library will have the method for preparing the really hard-to-get essential precursor. Or you can buy it (it's not cheap for laboratory grade stuff) from a chemical company.
You can make sarin and try to store it--but it breaks down over 6 months or so. Impurities in the precursors, failure to eliminate some by products, letting things like moisture into the chemical, all speed the breakdown of sarin and can reduce its shelf life to a few weeks. Or you can just have a chemical shell that keeps the two components separate but allows mixing just before use. If you'll recall, at one point they found such warheads, such shells in one of Saddam Hussein's arms depots.
The first round of chemical weapon claims last year involved sarin in the same area--and that's probably undisputed now. What was disputed was what Russia stated--the impurities in the sarin didn't match what would have been in the Syrian government's sarin. It was of different manufacture. Those impurities are unlikely to break down as fast as sarin. And, in any event, the sarin after it decomposes still leaves traces of reaction productions.