https://www.geoportail.gouv.fr/carte select the Cartes IGN Classique.
https://www.utagawavtt.com/search?city=&w=%5B1.09549,44.78640,1.47314,44.95137%5D&q=%5B1,2,3,4%5D&k=0&l=all&u=1&af=7838&aa=10
Double dashed lines are a fairly good indication of tracks on the ground. Solid black ones are an indication there might be one if it hasn’t become blocked over time.
I use Strava Heatmaps to see if there’s any recent activity.
I found some good, a lot of rubbish and lots of boggy and blocked – but that’s in the Auvergne which is a bit more sparsely populated.
Since using heatmaps I’ve had much more success. Use streetview at the junctions of tracks and road where possible and it will give you an indication of whether it’s worth it or not.
If there’s no signs saying don’t ride I tend to ride it until I get stopped and then give a load of gallic shrugs and “Perdu”
Oh and I will be a couple of hours east of you next week in Sunny Pleaux!