I'd eyeball it on one of the OSM-powered map sites, like
Bike Route Toaster
or
CloudMade Maps. If the roads are all present, they'll be all on the GPS and I'd have full confidence that they were up to the task.
If there are gaps and they're minor - connecting roads between two little villages where it's going to be just a mile of farm track - I'd go with it. I've found over the last year that OSM is fine for getting you around: at worst, you turn off "follow roads" and just make turns according to the big arrow.
(I've trusted it with white-outs on Dartmoor paths, so I'm either a lucky fool or the risk is way lower than people think)
When we did Dumbarton - St Andrews, there were two great wodges of unmapped road. That was
very minor rural stuff; anything bigger, especially in England, is all done by now.
Eyeball it on the toaster first, and you'll be golden.
![Thumbs Up :thumbsup:](/forum/Smileys/classic/thumbsup.gif)
Edit to add: Since audax wants you to follow a specific route, I'd always take the route sheet along. It's often got handy extra info, and takes up no space in a jersey pocket.