Sarin is an organophosphate that irreversibly inhibits cholinesterase. it’s a neurotoxin, and a potent one. It’d be absolutely terrifying as a weapon, if it weren’t so unstable. Even if a rogue state had gobs of Sarin last year, it’s all pretty much a dud by now. The instability of acid halides (carbonyls there, but all of them in general) pretty much ensures this. So the civilized world has that going for it, which is nice.
How do you test for sarin leaks? With a rabbit.