My very first choice is to use routes and maps from Adventure Cycling Association. Their routes are selected by experienced bike tourists who are often local to the areas being mapped. Their maps will include a list of bike touring friendly services along the way.

Next up, is google maps with street view. I pick a likely looking route and switch to street view mode to look at sample sections. From the street view I can see the nature of the road, how much shoulder area (if any), and get a very rough idea of the traffic by observing how many other cars are generally in the picture.