If you are wondering whether the mushrooms in your lawn are poisonous, assume that the answer is yes until you learn otherwise from a professional. Never try to figure this out on your own. Only a mushroom expert can determine whether mushrooms are poisonous. Never test a mushroom yourself; poisonous mushrooms can be deadly.
You may want to determine whether mushrooms in your lawn are poisonous so that you can decide how aggressively you need to treat a mushroom invasion in your lawn. You do not want children or animals to eat lawn mushrooms under any circumstances. If your mushrooms are poisonous, you may decide to actively pursue controlling them. Whereas, less harmful mushrooms can actually benefit the growth of other plants in your garden, as they help organic matter decompose, releasing nutrients for other plants.
There are no tests that you can use to figure out yourself whether your mushrooms are poisonous. Contact your local university cooperative extension for advice on identifying your mushrooms. They may have an expert who can help you, or who can direct you to the appropriate resources. They can also tell you the best methods for controlling invasive mushrooms in your local conditions.
In general, it is very difficult to totally control the growth of fungi in a lawn. Once it finds conditions where it can thrive, it spreads rapidly. However, you may be able to deter the spread of questionable or unsightly mushrooms by picking and discarding them. Serious problems need more active solutions, such as digging up and removing as much of the mat of fungus in the soil that is producing the mushrooms, then reseeding the lawn. You can also apply a nitrogen fertilizer to help create adverse conditions for mushroom growth. In most cases, applying fungicides to your lawn to kill mushrooms is less effective than using manual controls.