i've researched 'Charlie Charlie' it's a made up hoax. what is happening is you've subconsciously convinced yourself [or one of your friends is playing a prank on you]
pendulums, automatic writing, ouija boards and other similar divination tools where the spirit communicates through you will always have some level of doubt for several scientific reasons, so you should come to your own conclusions if you believe it or not.
regarding how to tell, whenever i'm communicating with a spirit through a divination method i can feel their presence. it depends on the spirit really, but i've felt a spirits hand on my shoulder, a warm feeling [like i was being hugged] fear, confusion, a hand on my hand, a hand on my head, or a cold feeling. i use to be able to hear spirits most of the time so i could sometimes hear they say the answer before the pendulum or whatever moved [and it doesn't sound like it's a voice in your head, it would sound like there's someone in the room with you] of course this is different for different people. my friend who works with pendulums says she gets a strong feeling from the spirit, like i'd ask a yes or no question and she suddenly gets a strong sense the answers a yes, like from a memory.