system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do people determine how fires start? Wouldn't all the evidence of the cause of the fire be destroyed in the fire?
gpt
If you use a chemical, something that burns easily (an "accelerant"), like gasoline or acetone or something like that, it can leave behind traces in the remains of whatever was burned, like in the soot or charred remains. Those remains can be analysed in the lab and traces of accelerants discovered that way.

Also, fires that are deliberately lit can be distinguished  by their starting place. For instance it might be quite suspicious if a warehouse fire started in all four corners of the warehouse simultaneously. You can determine where a fire started by looking at the damage it did, for instance Light bulbs tend to melt toward the heat source, so the "direction of melt" can indicate the direction the fire took.

As for the cause of an accidental fire, electrical problems are the most common cause. By examining a cord, or a fusebox, or an electric outlet, an expert can usually determine whether it was burned BY the fire, or was the SOURCE of the fire. Consider an electric outlet. The metal parts, for instance, probably don't burn but are charred, the plastic parts may melt from the heat, and the pattern of charring and melting can indicate whether the fire started inside the outlet or outside. The way the copper wiring melted or "splattered" could indicate an overheated circuit or an electrical short.