Based on smell.
Sulfur mustards are viscous liquids at room temperature and have an odor resembling mustard plants, garlic, or horseradish, hence the name
https://en.wikipedia.org/wiki/Mustard_gas
One of my relatives got hit with it in WWI. As a result, in later years, he went blind and had lung issues that ended up killing him way too young. He (and apparently others in his company) had to fight to get disability.
I was told the enemy would first fire a gas that would induce vomiting so soldiers would have to take off their gas masks to puke just in time for them to fire the really bad shit. I haven’t verified that detail though so maybe it got mixed up hearing it third hand. He died years before I was born.
From german wikipedia: It was a practice called Buntschießen, ~ multi-color shooting, as German chemical weapons were color-coded. One would start with “Maskenbrechern” ~mask-breakers (blue) , designed to force soldiers to remove their masks, which were then followed by lung-affecting agents (green).
