Look for an arc or circle of lush green grass and/or toadstools or puffball mushrooms. The rings may be as large as 60 feet in diameter, though most are less than 15 feet. They usually occur in the same place each year, with the ring expanding outward.
The lush grass is from a release of nutrients (especially nitrogen) caused by the fungi. Some of the mushrooms associated with fairy rings are poisonous and should be raked up and removed regularly if small children play in the area.
Fairy rings are more of an aesthetic nuisance than a threat to lawns. Sometimes there will also be a ring of dead grass caused by the fungus. More frequent mowing can minimize the difference in grass height. More frequent fertilization and heavier watering can even up grass growth and color, but may worsen disease problems.