Residues of known chemical agents are pretty easy to detect. Of course, you can't know from that who dispensed the agent. The nerve agents developed in WW 2 are plenty deadly, Tabun, Sarin, among them. Hitler had a stockpile and never used it in the West.
and this prompts story telling time:
some 20 years ago i fielded a network of chem/bio sensors in the middle east. these were stationary devices that communicated with a radio modem- some high speed mess, at the time.
the chemical detector first implemented utilized an ion mobility based tech.. it was decently accurate but finicky to maintain especially in that environment with fine dust constantly loading up the filtration. at any rate, i had two that would constantly trigger, and they were adjacent each other. i never encountered a 'network' alarm, just individual units alarming... the way the algorithm worked was based on wind speed, barometric pressure, wind direction, ect--- a 'real' even would plume and that plume's movement was directed by meteorological events which could be anticipated... if a sensor fired off and then another a known distance and direction away, it would be considered a network event and the response handled much differently. these issues i faced weren't 'network'.
so, it was ion mobility as i've already stated... come to find out, the mathematics worked out that JP4 was close to a particular nerve agent in it's behavior as it passed through the chamber. it is nowhere near or even related to it molecularly- but in the case of ion mobility, it acted similar. weird, i know, and it took me some time to figure this out... bounced it off the white coats when i had enough data to support my discovery and it was no surprise to them... they looked at me dull eyed and said "yeah? what of it?"...
point being, yeah- if you have a sample and especially if that sample is not contaminated, it's 'easy' to identify even in the field... but, it's just as easy to go off on a wild goose chase and think you have something you don't if you don't carefully reconcile through different means/techs of identification.... in the field, mind you.
methyl parathyon is an example- it's a pesticide used sometimes in crop dusters. Ukraine is heavily agricultured... if an explosion struck a storage facility of that, alarms would ring as if it was a nerve agent.... because, it is. just not one that would hurt humans unless concentrated. i'm not making an excuse for the reports- i don't know... nobody does right now... i just hope this is carefully considered before action in response it taken... if they used chemical weapons, it's a game changer.