Setting aside the debate about the "calorie content" of mushrooms, the main reason to harvest them is that they are DELICIOUS! At least morels, chanterelles, and portobellos are and all can be found in the wild.

If you are interested, there are a variety of detailed guidebooks available that discuss the specifics of identification.

The first step is to get familiar with the deadly mushrooms.

Then learn what type of edible mushrooms are in your area and then see what deadly/poisonous mushrooms resemble those and then see how to distinguish them. This is a decent starting point for the western US (http://www.amazon.com/All-That-Rain-Prom...2622&sr=8-4).

If you don't know what you are doing, don't eat wild mushrooms. And, by the same token, if you don't know what you are doing, don't drive a car, fly a plane, operate an arc welder, go boating, go swimming, etc.