Check for mushrooms. Where certain kinds of mushrooms grow, the grass changes colour (becomes lighter or darker) because the mushrooms absorb many of the nutrients in the soil. Mushroom spores spread in a circular fashion and as they feed what is commonly known as a "fairy ring" appears.
One mushroom, the Marasmius Oreades (fairy ring mushroom) is an edible variety and often the first to appear during the spring in North America. Of course, you have to know a great deal about mushrooms to be sure that any you pick are edible rather than poisonous before putting them into your omelet. If you don't really know the difference, better to pick them from the shelf of a store than risk dying over a mushroom.
When I see a fairy ring, I part the grass to look for tiny mushrooms. Not to pick them, just to see them. There are so many varieties of mushrooms, so many odd shapes. The root systems of mushrooms can be hundreds of years old, extending in every direction for acre upon acre, though you can't see any of this other than the small "flowerings" of individual mushrooms standing above ground.
The magic of a fairy ring lies in what's underneath it.