It is very easy to mistake poisonous mushrooms for edible ones, therefore only very experienced foragers should attempt to pick wild mushrooms. In any case, do your research very carefully before setting out. The best way to go about picking wild mushrooms is to learn about a few edible species and stick to picking just those, as well as any poisonous varieties that they may be easily confused with.
Pick a mature mushroom out of the ground and tear it apart to see if there are worms in it. If there are, there is a better chance it is edible; if not, it might be poisonous, as the worms know better than to go near it.
Smell the mushroom. If it smells unfamiliar--that is, not what you would expect a mushroom in a supermarket to smell like--then it's best to leave it alone.
Add garlic and the picked mushrooms into a frying pan with oil. If the mushrooms start to turn black, don't eat them, as the garlic reacts with the toxins in the mushrooms to change their colour.
Using pen or pencil and paper, make a note of the mushrooms you come across that are edible to save you time on future expeditions.
Write down the colour, texture, general appearance, smell, location and any peculiarities during cooking.
Look out for green-spored mushrooms with large caps which can be up to 10 inches in diameter. They are poisonous.
Check for red, orange or yellow caps with white specks on them. These could be "amanitas", which are poisonous and often grow under trees. The white versions of these are deadly.
Leave mushrooms with large flat caps that are red, purple, white and yellow and very brittle in texture. Some of these are edible and some are poisonous, therefore it's not worth taking the risk.
Also leave mushrooms that grow over decaying roots and on tree stumps. These could be "Jack-O-Lantern" mushrooms and are very poisonous.
If you're serious about picking wild mushrooms it's worth going on a course or taking along a very experienced forager to make sure you don't make any mistakes. If in doubt over any mushroom, always leave it rather than take a chance. While some just cause discomfort and a stomach upset, other types can be deadly.