The hallucinogenic mushrooms known as Liberty Caps grow throughout most of the UK, where they are usually referred to as "magic mushrooms" or "magics". They generally first pop up in mid October (depending on the weather; they like it cool and damp), and are most often found in areas where short grass meets longer grass, for instance on the edge of marshland, where they tend to be tangled in among the longer grass. For this reason they can be initially difficult to spot, although, as veteran mushroomers are fond of pointing out, once you've spotted a few they suddenly become much easier to find (this is absolutely true).
It hardly needs saying that some mushrooms are extremely poisonous, and therefore correct identification of any mushrooms that you intend to ingest is absolutely essential. Magics found in the UK have a couple of very distinct features which make them relatively easy to identify. Firstly, the stems are almost never straight; instead, they are usually markedly spindly and 'kinked', often zig-zagging two or three times along their length. This is a very unusual trait in mushrooms. But there is a further and even more reliable means of identification: if the stems of the mushrooms are crushed or 'bruised' just after picking (done by rolling them roughly between thumb and forefinger), the stem should develop a blue or blue/green tinge within an hour or so. If that happens, they're almost certainly magic mushrooms.
Please note that I cannot speak from personal experience regarding mushrooms found in other parts of the world, but I have been led to believe that most varieties of psilocybin-bearing mushrooms exhibit the blue tinge upon bruising. Without a doubt though, the absolute best way to be sure you've got the right kind is to learn from an experienced mushroomer.
Although some smaller mushrooms in the UK are mildly poisonous, it is unlikely they would kill if taken in usual mushrooms doses (typically 15-30). This is not true for all parts of the world though, and a small mistake could potentially be far more costly.
Let your bottom line be: if in doubt, leave it out.