Prove a negative? Start with common sense aspect though, why would the regime, full well knowing the sensitivity of the chemical issue, gas a measly 50 odd people. Iraq at least managed to gas tens of thousands with the chemical weapons we gave them in the 80's.
For the same reason Saddam used chemical agents on his own people. Middle eastern despots rule by fear.
The scale was quite different though, and the US was opposing Assad using any chem weapons, whereas the US had given Saddam his. Is it possible Assad, or someone in his regime made a gross miscalculation? Assuredly. But in the news articles I parsed, one thing that was glaringly lacking was actual evidence, or any type of reliable information from the ground. Instead, it was a bunch of opinion and "ohhhh the children" type emotional BS and clickbait. Very little detail on what happened or how, in favor of expounding on how horrible it was.
But then again crazy people don't think like everyone else. If they are innocent as they say I don't see the issue of them either letting people in the investigate. Otherwise it's perception. If you happen to be one to open carry and someone shoots another person and takes off and you are the only one around with a gun the perception is you did it. In which case once investigated would be cleared of wrong doing. Which is why the same thing should happen with Syria... if they don't allow an investigation they are implicating themselves.
But who would investigate, and how? If the area is rebel held, which it must be, otherwise why bomb it, how would Assad be able to facilitate an investigation anyhow? Plus, I have never seen anything to lead me to believe Assad is insane. He might not be softhearted, but that is different than insanity or irrationality.
I do have a bit of sympathy for the Alawite minority he represents. The Alawites and Christians in Syria have long stuck together because they are surrounded by rabid Sunni extremists who if not suppressed, would in fact slaughter and rape their way into a failed state similar Afghanistan under the Taliban.