They don't need to cover the whole forest, usually the fire is first seen when still small. But it's a lot of work, I don't know how often they are successful:

They don't need to cover the whole forest, usually the fire is first seen when still small. But it's a lot of work, I don't know how often they are successful: https://www.nwcg.gov/sites/default/files/publications/pms412.pdf https://www.nwcg.gov/sites/default/files/publications/pms412... (I also know that forensic fire investigation for house fires/criminal investigations is basically a made up art with extremely unreliable results - not sure how much that overlaps with wildfire investigations).