Make appointment to visit the "County Hall" appointed ROW (rights of way) officer, explaining the area you are interested in.
They will pull out the "Definitave Map" - this will be a O.S. map - highlighted with classifications - they will mark up if the road is with public vehicular rights.
Follow up your visit with a letter asking for written confirmation of public vehicular rights as viewed - from A to B to C etc.
Depends county to county - some are not too willing to maintain their statutory obligations - the "DEFINATIVE MAP" may be "definative map".
IMHO - more to do with local politics. 30 years ago - some National Parks Authorities actually published booklets and listed all the "green lanes" with vehicular rights. The Yorkshire Dales NP - did this along with surveys carried out by university students , detailing extent of damage and identification of type of vehicle doing the damage (tyre evidence , farm vehicles & forestry being the worst offenders) - not so now AFAIK , to good the documentary evidence they were providing & too many 4x4 nutters
