This is currently "news":
A senior Israeli military intelligence official said on Tuesday that Syrian President Bashar Assad used chemical weapons last month in his battle against insurgent groups.
…
In his assessment, Brig. Gen. Itai Brun, the head of research and analysis in Israeli military intelligence, told a security conference in Tel Aviv that Assad has used chemical weapons multiple times. Among the incidents were attacks documented by the French and British near Damascus last month."To the best of our professional understanding, the regime used lethal chemical weapons against the militants in a series of incidents over the past months, including the relatively famous incident of March 19," he said.
But what, if any, is the evidence the Israelis have?
"Shrunken pupils, foaming at the mouth and other signs indicate, in our view, that lethal chemical weapons were used."
These are only visual signs. Instead of doing a chemical analysis that would be required to find chemical weapons the Israelis seem to have simply been looking at videos provided by the insurgents in Syria.
"Shrunken pupils, foaming at the mouth and other signs" in videos, if genuine at all, could have come from a variety of causes. These symptoms could be a sign for heroin and other drug use. Medical issues like seizure could also explain these symptoms. Sarin exposure though leads to lots of additional symptoms and would likely also effect those giving first aid to, or is making a video of, anyone who was seriously exposed to that stuff.
In the "famous incident of March 19" there were no chemical weapons but chlorine that, according to a Reuters reporter, was used against a government held barrier. Several Syrian army soldiers died. The attack came after insurgents had captured a factory east of Aleppo that produced chlorine for disinfecting drinking water.
So lets call the Israeli claim what it is: Factless propaganda.