This is a very deceptive mushroom. It can look identical to Agaricus campestris, the common (and edible) meadow mushroom. But, this one is NOT edible. To distinguish it from A. campestris, put a drop of KOH (Potassium Hydroxide) solution on the cap or stem. If it turns yellow, it is A. californicus. The KOH reaction is very rapid with a fresh sample, but may take a few minutes with an older mushroom. The yellow can sometimes be seen just by scratching the cap or the base of the mushroom, but this sort of bruising reaction can be weak or absent in A. californicus. The most reliable field characteristic is the odor. A. campestris has a mild ‘mushroomy’ odor. A. californicus has a ‘phenolic’ odor which is much less pleasant and more chemically. Another indicator is where it is growing. A. campestris grows in fields (or lawns), as its name implies. A. californicus tends to grow in woods or in disturbed areas. [much of this entry is extracted from rpeters observation notes on specimens he found in Prior Lake, MN.]
