short answer: expiration date on the bottle.
The "N" I've got, its from veterinarian supply, so its an official "medication" to put cows and dogs to sleep, probably euthanasia or for surgery I'm not sure.
Since its an official medication, it has the expiration date in the bottle or box, dont remember, but many people here had the
I believe if im correct: Expired on March 2021? or was it 2020? cant remember for sure.... I dont want to open the black-taped-box its in, because light affects it, and they say some chemicals would build up inside the liquid when expiration starts or is set.