They will use multiple sources (if they have to) to arrive at your payout, its negoiated by an insurance adjuster that works for the insurance company. They may contact dealers, look at kijiji and autotrader, and use the inhouse blue book to arrive at a value. They are usually quite fair, sometimes not but settling is what they like.
If you have an unusual, vintage or rare motorcycle it may be worth having it appraised in case something happens, however you'll need to tell your insurance carrier its appraised value and may be billed accordingly.
I spent a decade in the repair industry, watched dozens ,bikes,cars,snowmachines go through the write off process. Cars and trucks are easy.