There are no general rules for spotting the dangerous caps. You get to know mushrooms in the same way you get to know people - by learning to recognize individuals. Don't try to learn it all from books. Books can be useful after you know mushrooms, but usually don't help you get to know them. It's much better to go out in the field with people who know what's growing in your area.
Species can vary widely from one region to the next.
Be gentle to the environment as you collect its bounty. Mushrooms are the fruiting bodies of a tangled web of fungal tissue, called the mycelium, that grows underneath the soil or throughout needles and rotting leaves packed on top. Fortunately, mushrooms seem to benefit from picking, as harvesting seems to stimulate more fruiting. But you can harm the mycelium underneath. Cut the stems of the 'shrooms, don't yank them out.