That is a valid concern. The ones I picked grew on wood, not sure I could bring myself to dig through cow poo for a trip. Anyways, here is a good resource for mushroom identification, which for some like myself, helped alleviate some of the concerns of Mushroom hunting. http://mushroomobserver.org
I've read posts online saying some species that grow on certain wood in California, have shown up in flowerbeds as far as the east Texas. Supposedly the brand of wood chips...not sure I buy this, but I suppose that is possible. Never have I found shrooms in my front yard, buuuuttt...without getting too anal about it, I can say technically I could find them in my parent's backyard, as they live on 35+ acres. In my area of east Texas, we have 4 species of shroom: 3 from the cows, and 1 that grows on moss. If I really wanted to find the moss species, more than likely I could over at my parents. However, there are fields everywhere in all directions from my house, less than a mile away. So, eh.
I just found this bit online, and started laughing...just read and you'll see what's funny about it. Amanita muscaria (fly agaric) /is/ psychedelic, but its active principle is different to psilocybin shrooms. One cap is sufficient for a hallucinogenic experience. More than that may make you ill. 15 caps could be fatal. The safest way to take fly agaric is like the Siberian shamans, by drinking the urine of someone (or a reindeer) who has eaten it; the active principle is still present in the urine without any of the toxic compounds in the mushroom. lmao! reminds me of Beavis and Butthead sticking a toad in their mouth after hearing you can get high from them.
I would not put Fly Agaric and Psilocybin mushrooms in the same category. Having done both the Fly is not nearly as psychedelic. Yes, something happens but it's more of a haze where as Psilocybin rips my third eye open.