Conventional smoke detectors for domestic and commercial alarms are readily available (in most countries) and use an alpha emitter (americium-241) as the detection method. The alpha particles ionise the oxygen and nitrogen molecules in the air of the detection chamber and set up a minute current flow. The smoke molecules neutralise the charged particles which alters the detection current so setting off the detector alarm.
There are many industrial forms of fire detection such as thermal sensors (resistance, bi metallic strip, fluid expansion bulbs, fluid fracture bulbs), infra-red detectors, ultra-violet detectors and no-doubt many others but it is best to detect early smoke, rather than wait until flames occur.