Often times you hear about the cause of a fire being a cigarette for example. However, isn’t the cigarette already long gone after the house/forrest/etc. has burnt down? How is it still possible find out what started the fire?
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Step one is finding where exactly the fire started. This is determined using a handful of methods, including eyewitness accounts, how much of the location was burned, etc. This kind of shapes the rest of the investigation, as fires starting in various locations are usually do to specific things. Like fires in the kitchen are either cooking mishaps if cooking was in progress at the time, or maybe electrical due to the large number of appliances, or gas leaks if the house used gas.
Then, we need to find out if the fire burned organically or if there were other factors. How charred things are is a rough indicator of how long it was in the fire or how hot it got. Usually, the seat of the fire gets the hottest, and things burn less and less as they get farther away. But if there are multiple really burned or charred areas, it *could* mean there were multiple ignition points, and therefore it could be arson. Note that this isn’t always the case, if a fire burns a house down and it looks like there was ignition in the kitchen and the garage, it could just be that the fire started in the kitchen and ignited a gas can that was already in the garage, but it is something that would be investigated.
We can also look for chemical residue, which is left by a number of things. If it was arson, and someone used an accelerant to start it, then maybe some of the original accelerant is left (possible if the fire doesn’t have time to really take hold or the arsonist applies it in a dumb manner) or it burns and leaves behind a chemical that wouldn’t normally be found. It doesn’t have to be extra chemical residue though, information about stuff that burned that was already there could be very important. Some stuff only burns at a certain temperature, so if we find that stuff burned we know the fire got at least that hot. Maybe that temperature would be impossible to achieve without an accelerant, which would be a strong sign towards arson, even if we didn’t find any evidence of the accelerant itself.
And lastly just common detective work. Sometimes we never really know, we just make educated guesses. It reminds me of Willem Dafoe in Boondock Saints sometimes, there is a decent chunk of science involved but at the end of the day it’s just finding the most likely cause and making sure that it is reasonable. Of course it’s way more complicated and there are countless other things that could be studied, but that’s a relatively short overview.
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