How do you tell if a mushroom is poisonous?
There are many different types of mushroom, so without getting a good grasp of individual kinds of mushroom it would be difficult to say whether one is poisonous or not. A general rule would be to never consume a mushroom unless you have a positive identification.
Some deadly varieties of mushroom include the death cap, destroying angel, Galerina species, small Lepiota species and the deadly webcap, and to be able to detect these you would need to have observed them all at different stages in their development. Having knowledge of where the mushroom has come from can help, too, as some mushrooms that are safe in Europe have deadly lookalikes in North America.
It’s wise to avoid old wives’ tales such as ‘a mushroom is poisonous if it tarnishes silverware or turns blue when bruised’, as these are both completely false statements. The truth of the matter is that there are no external easy-to-identify characteristics that all poisonous mushrooms have in common. To be absolutely safe, unless you are an expert mushroom identifier or mycologist, we would advise buying your mushrooms from a reliable source such as supermarkets, grocers or restaurants.