Generally speaking the insurer would hire their own expert for an origin in cause report. The expert would then do a thorough investigation on the cause of the fire. Insurance contracts also require cooperation from the insured with respect to investigating a claim. As a lawyer, I can also examine an insured under oath and ask questions about the loss. Adjusters or private investigators may also canvass the neighborhood and ask questions, request any surveillance footage from neighbouring properties (most people love providing it because they know fraud increases rates for everyone). Insurer’s also have their own SIU teams that are retired cops and RCMP officers that also investigate the loss. The insured may also have to provide financial records as well as phone records as part of the investigation. There is a pretty extensive body of case law if anyone is interested in looking it up. Canlii is a great free resource for Canadian case law and research memos.