In the past I’ve used a mixture of Google Maps with the cycling overlay combined with Open Streetmap and liberal doses of google streetview to check the path I’ve been directed down isn’t a tractor track across a field or a trunk road.
Google on it’s own is a bit crap though. It tends to overly favour paths that aren’t suitable on a road bike and also go the long way round to avoid a short section of unsuitable road where you could just bear it or walk the pavement. (I assume this is becuase it is meant for short cycle comuting in citys with cycle infastructure rather than long distance road bike tours)
Now though the Strava route planner is excelent and my first point of call.
It uses route popularity data so takes you where people like to cycle. Used it to get from South Bucks to Richmond, Surrey the other day to retrive a car after a night out. Google wanted me to go a lot of the way on towpaths which isn’t really feasible for a 50km+ ride in 2 hours. Strava picked a perfect route which I’d never have seen on the map but was as quiet as is possible when cycling towards a major city.