What properties would have caused early researchers to name hydrogen "inflammable air"?
Dihydrogen is colourless, odourless, and tasteless. How do we characterize it? How do we even know it is there?
The action of an acid on a metal usually produces such a gas:
The gas formed could be collected under certain circumstances. Its major distinguishing property was the reaction the gas underwent when a glowing splint was introduced to the vessel. The result? An audible whoosh-bang, and later careful researchers might have noticed the formation of water (and hence called this gas hydrogen, literally "water-forming").
So, the earliest researchers knew that the substance was a gas, but it was a gas unlike the air they breathed. Hence the name "inflammable air".