We don't have a smoking gun either way, but we can apply some critical thinking and view of the current strategic balance to figure out a 'reasonable' conclusion, right? So what do we know...
All Chemical Weapons admitted to by the Government (critical point) that were still in the hands of Government Forces and not Rebels (critical point), were removed and destroyed with UN verification. No evidence (demanded by Russia in the UNSC at the time, and not provided) has been shown that the Syrian Government has additional Chemical Weapon Stores or has ever actually used them.
Chemical Weapons HAVE been confirmed used in Syria and in Iraq. Syrian Forces do not operate in Iraq and those Chemical Weapons were directed against Iraqi Forces. ISIL and other groups have and do operate in both countries.
Syria faced a 'red line' threat from the then Obama Administration and a world wide outcry (certainly in the media) of consequences, while the Syrian Government and Russian Forces maintained their denial of Chemical Weapons use and blamed rebel groups. Tensions were high as everyone will remember.
Since the liberation of Aleppo, the fight has been going badly for the rebels, with the Government taking back control of Damascus' main water supply, Palmyra being once again recaptured and Turkey rolling back YPG forces along the boarder.
Politically it is the same with Russia, Syria and Iran establishing cease-fires with some rebel groups and the general impression from recent summits that the remaining hostile groups (and their backers?) have no political solution they are willing to accept other than total victory, however likely or unlikely that is.
We have a new US administration that came into power critical of past Syria policy and hinting that reality on the ground had changed the game.
i.e. regime change.
A few days later, we witness reported deaths in Idlib, linked to Chemical Weapons (just now being described in media as Sarin) after a Government Air Strike.
Western Anti-Government Forces and Media immediately (and I mean immediately) say it was a direct chemical bombing, with the Syrian Government squarely to blame. While the Syrian Government and Russian Forces in country say it was a standard bombing run, which must have hit chemical munition stores held by the rebels (whether bought, stolen, manufactured on site or otherwise obtained).
Now we are back to a world wide outcry for consequences (massive media campaign that none can deny), retaliation against the Syrian Government and removal of Assad as prime policy.
So with all of that said... is it possible Syrian Government Forces launched a Chemical Air Strike in Idlib? Yes. We don't know that absolutely all Chemical Weapons held by the Government were turned over.
Does it make sense for the Government, if it does still have some of these weapons, to use them in the current strategic environment and after the last round of outrage? No. Not to my mind anyway. I've seen a number of interviews with Assad pre and post crisis. He's a Doctor, comes across as pretty intelligent, and not as a ruthless dictator like Saddam.
Is it plausible that Chemical Weapons were on site in the hands of the Rebels, either manufactured or supplied by their backers (Saudi Arabia perhaps?). Yes. This seems like a not unlikely scenario. Either by unhappy accident of location or planned intent, it surely plays well for the Rebels geopolitically... Just look at the media reversal going against Assad and what the near future outlook for the Syrian Government might be.
So what seems more likely to you?