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The point of origin of a wildfire can reliably be located by waiting for the fire to die down and cool off, then a fire investigator will reverse track the fire to the exact location where it started.
It's kind of like tracking the path of a deer's footprints to where it came from, except that a rain storm will not wash away the tracks. Fire tracks will last through storms and last for months.
There is also the factor that successful fires start small, and starts immediately burn up hill, or in the direction of wind, if there is wind, to find more fuel to burn. As it moves it grows larger and hotter, wider. The fire will move away from its source leaving what started it just slightly singed, not completely burned away.
Suppose a bottle rocket lands in some tall grass and starts a fire. Where the bottle rocket landed will be the least burned area in a much larger burn area. As the investigator walks toward the point of origin the path becomes more well defined and the width of the burned area will grow narrower until it reaches the point where the fire started, at which point the trail ends with a palm sized patch of blackened grass, with the visible remains of that bottle rocket marking the spot. The fire investigator can then pick up the blackened, but not completely burned bottle rocket, and take it to the fire lab.
In the lab they can test it for DNA, and perhaps a portion of a fingerprint. In a case where a cigarette butt started a fire there will be DNA in the unburned filter. A smoker has saliva, which contains DNA, and when they stick the cigarette in their mouth it leaves a lot of DNA for the fire lab to find and sequence.
Most arsonnest are caught by the time they start three fires. Of all the types of crime, arson is the easiest to track to the source to collect evidence, and then find the perpetrator and arrest them.
