Like any investigation it will be a combination of things and will result in varying levels of success.
There is a lot of science now behind determining the origin of the fire such as burn patterns, depth of char, evidence of flow path and temperature, etc that has been fairly well covered in most of these comments but also the interview of first responders, residents, witnesses, and such.
Often there are people who can give lots of valuable info such as where and when they saw fire, color of smoke or fire, noises, smells, seeing someone where they shouldn’t have been, progression of the fire (what was it like when the chief first arrived vs. 20 minutes later?) and much else. Once all the available info is collected and looked at as a whole investigators can put together what happened and why.
There is an entire education and career path for fire investigators. While it is ultimately the fire chiefs responsibility to determine cause and origin of a fire that task will often be delegated to an investigation team with specialized training.
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