What is invaluable for planning is Google street view. Once you've established the route you can see what the shoulder situation is, familiarize yourself with navigation cues, and of course determine if your route is in fact a gravel one.
This forum has helped me many times too. Once I've established a route I throw it out there for advice and always get knowledgeable and helpful answers...even for Europe trip planning....struck out with Hawaii route advice though. Most recently I put out the "Ride across Nebraska..." question on this forum a few days ago and already have some helpful replies.
Just remember, planning is important but you'll still end up with some surprises and these, in my experience, are the best memories.