Agaricus Campestris, or the common meadow mushroom, has pink gills when immature, shading to dark chocolate brown at maturity. It's closely related to the small white mushrooms that are sold at grocery stores. The spore print is also dark brown. If you pinch or scratch the flesh and it turns colors (usually red or yellow) it's not campestris but some other kind. Other varieties also have a distinctive smell when crushed, but that can't be relied on since some people can't smell it. The smell is called "phenolic," whatever that means.
Eating related varieties that aren't edible will give you the runs, or make you throw up (or both) but is not technically dangerous. Another edible variety in the same family is the horse mushroom, which looks similar but is much larger. They're harder to identify. If they stain yellow but don't have pink gills when young, they might be a horse mushroom.
I had them for breakfast this morning. :) Fantastic in an omelet.
I'm going to try to start them under the grapevines.
