Photographs or other instruments can tell the difference between a supermoon and ordinary full moon. The supermoon of March 19, 2011 (right), compared to an average moon of December 20, 2010 (left). Image by Marco Langbroek of the Netherlands via Wikimedia Commons.
Size comparison between March 2013 full moon and May 2013 supermoon via our friend Alec Jones. Thanks, Alec! The difference in size is due to the moon’s location in orbit. The moon’s orbit isn’t circular, so its distance from Earth varies.