When learning to identify Monarch butterflies, it is important to know the markings on Viceroy butterfly. The Monarch and Viceroy are a great example of coevolution. The Viceroy has evolved to look more like the Monarch because predators avoid Monarchs due to the bad taste. This has increased the fitness/survival of the Vireroys.
Note the black lines at the tip of the arrows in the picture below. This is the trademark of the Viceroy, and a great way to tell which butterfly you are looking at.