....only way is to KNOW what each variety is, either by studying the subject or as I do make field trips with friends who KNOW.....guessing is not to be recommended as some of these things are really nasty.....
There is no way of knowing, I agree with the others, unless you are an expert. One of the most deadly, the Death Cap, looks quite innocent. Going on 'forays' is the best way . www.capsandstems.com is a good website and there are lots of good books on the subject. Also may I recommend the Association of British Fungus Groups www.afbg.org which I think can find groups in your area.