Last time the claim was helicopters dropping chlorine. Helicopters fly low so radar coverage of Russian systems cant prove anything flying or not flying. Hopefully gps recording and continious camera survaillance at least recording for several hours is attached on all Syrian helicopters to immediately prove that all of them are on the ground since otherwise the missiles will fly by dozens this time without waiting.
Better Russian af do any future coordinated operation on nusra and Sy forces can use imported armed drones or close range strikes by airforce,artillery to defend troop positions and advance by smaller steps. Russian radar coverage should record all air traffic for several hours. Russian gps recording the flying Sy, Ru aircraft can corralate with radar data and give better proof that Syrian planes are not flying nearby.
Simply claims of chemical attack is nonsense or illogical by Syria or its allies wont stop a usa attack as it didnt previously. After previous incidences Syria should be more creative about thinking the precaution measures similar or better than above to disprove the claims almost immediately even if its allies cant or wont.