Many of you who have spoken with me at the two area markets where, unsurprisingly, unaware that mushrooms should not be consumed raw for two important reasons: the cell walls of fungal tissue is a form of chitin, similar to cellulose, and even more importantly, many delicious mushrooms do have irritant compounds that can cause some real gastric upset if you do not properly cook them. At any market in Kentucky, you will probably not encounter truffles raw, which are probably the only fungus that is sold commercially fresh and eaten raw for flavor and not for any particular nutrient.
All other mushrooms you purchase or forage, however, should be cooked completely before enjoying in your favorite recipe. I cannot stress enough how this is extremely important.
It would seem some in the button mushroom industry (through perhaps no fault of their own) have perpetuated the myth that consuming their product raw is a good thing, and for most people, no ill effects have been encountered. Read here to understand why you should always cook.
You can, however, sample a mushroom (if someone allows you to do so at your favorite market) by taking small piece, chewing to ascertain taste, which sometimes is a good indicator of how “good” a particular species tastes to you, and using a napkin to spit the piece out in a safe and sanitary manner.