Open your closet, don't turn on the light. Make sure you have one match with you. Step inside and close the door. If the lights outside of the closet are on, this will not work. Nor will it work if it is daylight. You only need enough room for some slight mobility.
Stand in the darkness for about two minutes, since that's all that's needed. Now, take the match and hold it in front of you and say, "Show me the light or leave me in darkness." If you begin hearing whispers, light the match immediately. If you don't hear anything, and the match doesn't ignite on its own, don't turn around. If you're too late with lighting the match, or you don't do it at all after hearing the whispering, something will grab you from behind and pull you into what seems like an endless fall into darkness.
If you do manage to light the match in time and nothing happens after, open the door slowly and get out, then close the door but do NOT look inside. From then on, never look in your closet without the light on at all. Some say if you leave your closet open during the night you can see the demon watching you, with two red eyes that glow like matches.