1 John 4:1-6, Testing the spirits – Who’s behind the beliefs?

“Test the spirits.” A snippet of a verse that we all know. What on earth does it mean? Is John pointing out some extra-sensory perception that special Christians have? Are believers to tune up some special, spiritual gift-given antennae with which they can discern the unseen world of demonic forces and beings? Or is John pointing out something more more mundane, something that is the routine duty and capability of every believer?