system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can they tell that a huge wildfire, burning hundreds of thousands of acres, was started by a single campfire, or dropped cigarette?

Surely whatever evidence was there is long gone?
gpt
Fire investigators can tell which directing fires burned. With that knowledge that can get pretty close to the point of origin. Once there, they do some investigating. 

I live in California and the Rim fire WAS suspected of being started by Marijuana growers. That's what the news said but fire officials have stated the origin is very steep and would not be a good place to grow weed. 

One scientist has said it's possible the fire was ignited by a rock slide. Two rocks slamming into each other can cause sparks and it's been pretty dry up here.