The following are some of the signs that firefighters are looking for when they attempt to determine whether a flashover is likely to occur.
* Fast dark smoke.
* The neutral plane is moving down towards the floor. In this situation, a flashover is plausible.
* All directly exposed combustible materials are showing signs of pyrolysis.
* "Rollover" or tongues of fire appear (known as "angel fingers" to firefighters) as gases reach their auto-ignition temperatures.
* There is a rapid build-up (or "spike") in temperature due to the compound effect of rapidly burning (i.e., deflagrating) gases and the thermal cycle they produce. This is generally the best indication of a flashover.
* The fire is in a ventilated compartment, so there is no shortage of oxygen in the room.
Firefighters memorize a chant to help remember these during training: "Thick dark smoke, high heat, rollover, free burning."
The colour of the smoke is often considered as well, but there is no connection between the colour of the smoke and the risk of flashovers. Traditionally, black, dense smoke was considered particularly dangerous, but history shows this to be an unreliable indicator. For example, there was a fire in a rubber mattress factory in London in 1975 which produced white smoke. The white smoke was not considered dangerous, so firefighters decided to ventilate, which caused a smoke explosion and killed two firefighters. The white smoke from the pyrolysis of the rubber turned out to be extremely flammable.