http://www.landyzone.co.uk/lz/f13/finding-gree-lanes-what-i-do-80111.html
Further to that .. it's pretty easy with memory map, I use it all the time. I start with large scale road map, zoom in to an area I want to find lanes in, then draw a freehand rough square around the area using the track tool.
Then zoom in to a 1:50k map, then zoom right in to top left corner so that all roads can be easily seen and you only have four or so grid squares vertically. I find this is the largest resolution that's small enough to spot potential lanes, and large enough to cover an area quickly. Depends on monitor etc, on my large monitor I can cover a larger area, but on the laptop this is about right.
then use the cursor keys to scroll right until I find an area that might be of interest, lots of contours, few main roads a few minor roads, hopefully some green lanes.
Then right click and in to larger scale map to change to1:25k, zoom in and see what lanes/roads/features are there .. So now it looks like there are a few lanes, in this case not green lanes with a cross but unfenced lanes, marked with dotted lines, that are worth checking out. Mark the 'potential' lanes as a track in a bright colour, put your cursor over the start of a potential lane and get the grid reference written down to refer to the local council with.
If I live close enough I'll drive out and check them, they may just be someone's drive and not accessible to public, but I'd also try to find out via the local council website, Parks authority if it's in a National Park, or even use local newspapers to find out who owns the land and send 'em a letter! If I find they're legal to drive, worth driving etc I mark them a different colour, then when I actually drive them and find they're good, bad, whatever I change the colour again to green for good, with notes in the route/track properties so I can refer to them later.
Then carry on, right click and back out to the larger scale as it's easier to spot areas on, scroll right till I get to the edge of the box I marked, then scroll down a few grid boxes and start scrolling left to continue the process ...
Time consuming, but works. Only thing it doesn't do is give you the local knowledge, that's where going laning with someone who knows the area helps.