Before technological advancements allowed scientists to develop gas detection equipment, people had to rely on other warning methods.
Using your senses
Most gases are colourless and invisible to humans. However, smell and sound can help alert people to a gas leak. Some gases smell and some have a smell added to them, so they draw attention when leaking. It is also sometimes possible to hear a faint hissing from a leaking gas pipe.
Canaries in the mines
Canaries were taken into coal mines as an early warning system for high levels of gas. The canary would show signs of distress, cease any singing and eventually die. These signs would alert the miners before gas levels became a danger to them.