Look for the "light"...
I'm always talking on my bluetooth headset while I paint at work. People who show up to my job site sometimes wonder if I'm talking to them, or if I've just "lost it" and have started talking to myself. I always tell them, "look for the light on my earpiece...If it's on, I'm talking to someone...If it's off, I'm talking to myself!
I think I'm going to start using the same "test" on all of the many people in this world who claim to "hear from God". If I see "light" in their lives...love, kindness, generosity, compassion for the needs of others, selflessness...I'll be consider the fact that they may be "talking to someone". If I don't see any evidence of "light"...I'm pretty much going to think they are "talking to themselves"... It's not "fool proof" but it is a start.
36Therefore, if your whole body is full of light, and no part of it dark, it will be completely lighted, as when the light of a lamp shines on you." Luke 11:36 niv