1) Secret locations
Many local contractors keep their physical location secret. They rent UPS mail boxes with addresses that resemble real local locations. Always perform a street-view map search of their physical address.
Many local contractors are in the business of selling leads. Once they land a client, they simply sell the job to the lowest bidder. Ask if the work will performed by directly employed workers.
3) Find out if they are in trouble with the state
This handy WA state website shows you the legal status of your home improvement professional: