Here in timber country, sometimes the only clues are slight changes in the way the timber grows on old, disturbed soil... Infrared images are particularly helpful in that respect.
I remember several years ago having some time to kill and found myself in the public library looking at a book of photos of the English countryside taken from 1000'. The detail that came out of those photos was remarkable; old roads, villages, walls, buildings, etc.
Sounds like real interesting work.