People get really excited when they find mushrooms in the wild, don't they?
99% of magic mushrooms you find in the wild will be one of the following four:
Cubensis ("Cube"/"Cubie"/"Golden Teacher")
The cubensis has a white stem, golden cap, and dark purple gills. Very often you will see them having a prominent "nipple" in the middle of the cap. Good ways to be sure you've identified the cubensis are that it has a purple 'veil' (see first picture link), any part of the mushroom stains a bright blue when you pinch it (not brown, not black), and will almost always be groing in cow manure. You can sometimes find them in horse manure, but not often.
To be absolutely sure you have a Cubensis, you should take a sample mushroom home (not the whole lot), and set it gills-down on a piece of paper or tissue. After a day or so it should leave a dark purple print on the paper. If it's rusty-brown, it's poisonous. If the mushroom dropped its spores before you could get it to paper, you should just be able to tell by the color of the gills.
Psilocybe Cyanescen ("Cyan"/"Wavy Cap")
Have golden colored, floppy tops, brown gills, and white stems. They grow in wooded areas, usually have slimey/sticky caps, and stain a bright blue when you pinch them.
Psilocybe Semilanceata ("Lib"/"Liberty Cap")
Most often found in areas where short grass meets longer grass, the stems are almost never straight; instead, they are usually markedly spindly and 'kinked', might stain blue upon pinching but not always (low psilocin content), caps look like 'witch hats' -- very pointy.
I've never personally found Lib's because they don't grow in my area, but they're very common in grassy areas of the Northern US, and all over the UK.
Copelandia Cyanescen ("Cope"/"Hawaiian")
White/gray 'bowl' cap, white stem, black gills. Grows in tall grass, usually (almost always) where cow manure has fertilized the ground. Bruises a bright blue (high concentration of psilocin) upon pinching.
Not all mushrooms containing psilocin/psilocybin will stain blue, and not all mushrooms that stain blue contain psilocin/psilocybin. Do not take mushroom collecting lightly, because there is a good chance you will end up sick, or at worst, dead. Yes, there are mushrooms out there which will kill you from eating a single cap.
I ate a poisonous gym once back in my early collecting days just because it stained blue. I ended up puking my guts out with no 'magic' effects.
If you're not experienced on the matter, you should bring somebody along who is to help you. If you can't do that, you should join www.shroomery.org
, and post pictures of your finds to ask for confirmations about whether or not you've found anything good or dangerous.