First, turn the mushroom over. If it is an Agricus, the gills will be a rosy clay color when young, eventually turning into a brown color when older. Next disconnect the stem from the cap to see if the gills are free from the stem—if so, continue on.
To find the spore colors take the fresh cap of the mushroom and place it on a white piece of paper and let sit so that the spores can settle onto the paper. If the spores are a chocolate brown you are most likely in possession of an Agricus!
Agricus houses many species of both poisonous and edible qualities, yet each one has a special combination of features that give the key to its identity. For the yellow stainer, when the stalk is crushed it gives off an unpleasant smell of antiseptic (like phenol) and when the flesh of the mushroom is bruised it turns yellow. Yet the most defining feature is that when the tip of the stalk is cut it is a very bright yellow, much more so then any other part of the body of the mushroom and more so then any other Agricus.
The Yellow Stainer when cooked smells and tastes horrible and if eaten gives you gastrointestinal distress—overall not a very nice mushroom, yet fun to find. Happy hunting!