I was a fire investigator for 6 years. We go about finding what started a fire by A) looking at burn patterns and B) interviewing the people that were there if possible.
If the whole house has not burned down, I look for the that has the most burn damage.
In a urban area where the fire department is pretty responsive, you may only get smoke damage in most of the house.
This rules out the rest of the house from starting the fire. Let’s say the most burnt area is near the kitchen. You start asking yourself what can start a fire in that area.