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.