The underside of leaves are usually lighter in color and the wind will turn them over when it blows. The wind is coming from the light side of the tree. The length of the landing area can be determined by a flyby at a known speed and timed. At 100 miles an hour your are passing over the area at almost 150 feet per second. The main factor though is the condition of the landing area.
Tall grass can look much smoother than it is.
Rocks look smaller than they really are. Grass can also conceal mud and standing water. A pass over the runway doesn't always clear it of animals. A cow or horses might run if buzzed low enough, but a herd of cows or pigs can be a much bigger problem. After 4 or 5 passes with no success, if there's no help on the ground, you might as well go back home.