Not sure about the plowed field deal, there aren't many of them in the parts where I hunt and fish.
Train your eye to notice little differences in the soil, the rocks laying around, even at times what plants are growing in an area. Anything uncommon is a sign to keep your eyes open. Kind of like noticing an apple tree, a pear tree, and a rose bush in the forest means there's likely an abandoned homestead around.
While looking for tracks and sign, I noticed this was a bit different color than surrounding rock and dirt:
This is what the other side looked like when turned over:
Someone painted them lines on there and it likely wasn't no White Man.
Good luck, I hope your book search is fruitful.