Tall trees and mushrooms - http://morrie2.wordpress.com/2010/05/13/identifying-a-field-mushroom/ - has an excellent post on ID of field mushrooms of the various Agaricus species. We had some in supper tonight (and have been eating them fairly steadily, and I have a large fried and frozen supply, and about 2kg (wet) - or about 300 grams dry in the pantry cupboard. Having read up extensively, and talked to and show specimens to locals, I made up a cheat sheet for myself -- I recommend the Tall Trees and Mushrooms guide for more detail and pics, but for print-and-take-with-you... here is my personal cheat sheet. Understand this clearly: Eating wild mushrooms is dangerous passtime, and you do it at your own risk. Some can kill you, and some can have serious consequences. I'm not responsible for your decisions, you are. Remember: if in doubt - throw it out. Always ask locals advice and try to find someone who has real expertise to guide you.
My friend Morrie2 over at Tall Trees and Mushrooms (who knows more about this subject than I ever will) reckoned most of these sensible - except the first, as he reckons that would exclude a lot. Well, I live in a place of big paddocks, so it works for me. This advice (and it advice NOT a safe guarantee) applies here. I don't know about the rest of the world.
1)location - is open grassland/field - at least 10 metres from trees. Don't even look under trees or anywhere near rotting wood. (I know this cuts out a lot of edible mushrooms and a lot of area. At this stage my idea is over-cautious is best. We have lots of big paddocks here.) if it is growing in a cowpat, or moss or rotten wood leave it. You want grass around it.
2) how big is the mushroom, and what colour is it? If it is smaller than 5 cm cap ignore it. If the colour is anything other than a shade of mushroom-white (white with overlays of grey or brown) ignore it. (this hopefully takes out the galerinas.) And definitely ignore green shades (especially with white gills - Amanita phalloides - deadly - which shouldn't be growing out here anyway)
3) clear grass around the base of the mushroom to see if it has a volva. If it has leave it!!!! (Amanitas have volvas and A phalloides is deadly and shouldn't be growing out here anyway. But never take chances.)
4)Is the cap dry? good. If shiny/sticky, leave it.
5) Turn it over. Do gills go down the stem? - reject - if not: What color are the gills? If white or yellow or orange reject. If pink keep and see if they turn brown, if brown, then
5) Smell it. If it smells mushroom/anise-mushroom, good. If 'chemically' reject.
6) then check for an annulus (it must have one), check if the stem snaps cleanly from the cap, check if it peels easily.
7)back home check spore print - it should be brown.
8) cut and check staining. If yellow, discard. Pink or no stain is fine.
9) cook. If it smells chemically when cooking (indian ink/phenol) don't eat it. Eat one small slice. KEEP the rest of the mushroom unsliced, for ID if necessary. Do not eat any other sorts of mushroom. Wait 24 hours (at least) before eating more. If you get sick in any way - take the mushroom and yourself post haste to medical assistance.
Which all seems a great deal of hassle for some mushrooms - but they are very delicious and can be abundant and free. But be careful and read up and learn to recognise the toxic sp. (they don't look that like the edible field mushrooms, but people still make mistakes.)