They (and the police) tend to know what arson looks like, since they've had quite a bit of experience with it. People trying to collect insurance tend to do things like moving their valuables out of the house before they set it alight; things of that nature.
Oh, okay! That’s interesting. And let me guess, they show signs of tidying up personal affairs and sometimes go out looking at houses, cars, and other expensive things in anticipation of the payout. Another dead giveaway: telling a debt collector with certainty, “Get off my a**, this time next month you’ll have your money and everybody’s happy.”
But it was supposed to be an arson. By their neighbor.