Arson investigators are very well trained to look for the cause of the arson and the accelerants used to set the fire.
If the fire is put out before it completely destroys the building then it's much easier to determine the cause of the fire and the accelerant used in the fire.
Arson investigators determine accelerants used to commit the arson by looking for pour patterns and intense localized burning in one area.
Some people use gasoline, disease, and anything else that they can set the fire with but most commonly they use diesel or gasoline to set the fire.
If the person who committed arson only intended to burn the structure down or burn the land then the person convicted of arson can spend a minimum of 5 years in prison.
If the person who committed arson intended to cause harm or death to the people in the home or building they can get 15 years in prison or even 25 years to life in prison for the arson with intent to kill or harm someone or people.
If you set fire to your own home or building to do insurance fraud you could face 30 years in prison so never every set fire to your own place to try and get insurance.
The fire marshals and fire inspectors can find out through evidence as they are well trained to find out if the place caught fire as a result of arson or not.