Listen to Sue and wait until someone approaches you with the secret handshake!
Seriously, there is a very good thread at the top of this section on finding suitable lanes. Get yourself an OS map of the area that interests you
(Explorer 1:25, 000 is best) and look for lanes marked as BOAT (Byeway Open To All Traffic) and ORPA (other routes with public access) which are also known as UUCR's. These are probably legal to drive, but you need to verify that with your local Rights of Way (for BOATS) or High Department (for UUCR's)..
Additionally, sign up to Wayfinder and that is another good indicator. On the more popular lanes, people leave comments and these can be a good indication of how easy or difficult the lane is..That said, conditions on a lane can change quickly depending on the weather and traffic, so its best to go out with somebody else and also to walk any difficult sections on foot first..