It's dead easy to tell actually.
It had previously irritated me beyond belief but then I figured it out:
The tooltip that shows tile defense values - if it's 25% around the edge of a forest - you'll still get hit for 2 movement points.
(Or you can spot the 3 sprouting shrubs in the ground.)
Now we just need the pathing to calculate the shortest distance.
When I click 3-4 turns away - I don't want them to go through the forest taking 6-8 turns!!!!