How do we know that some poisonous gases are odorless? Do test subjects take a whiff of non-lethal doses and then we extrapolate that lethal levels are also odorless?
You could probably tell if a person killed by a lethal dose showed any indication that they realized something was wrong. I work in an industry that generates hydrogen sulfide gas as a byproduct. It has a very strong aroma, stench really, and can be detected by scent on a parts per billion level. The literature shows some variation in it, but certainly below 10 ppb. At very high concentrations, 400 parts per million, it can paralyze the olfactory nerves and kill without warning.
Most likely almost all odorless gases have been verified by the deaths of persons exposed to the gas.
CO i.e. Carbon Monoxide is/was a frequent cause of death due the the person not perceiving it’s presence before they slipped into unconciousness.
I’m pretty sure our knowledge of all such cases is from human experience – people who have been exposed but didn’t die.
Incidentally, it doesn’t necessarily help if the gas has an odor. Hydrogen Sulfide is notorious for having the stink of rotten eggs. It’s toxic, and you wouldn’t think that it could take people unawares. But apparently it’s impossible to tell the difference between low and high concentrations due to smell alone. You might think that you’re working near a low concentration (this is just a place that stinks, from the chemical work that used to go on here, you think), but really be subjected to high levels and be unaware until it affected you.