I wouldn't say there's a "test" in order to find out if you're talking to God. You'll probably just know when it happens. Of course comes the problem of believing your subconscious is acting as God, so you'll know, but you won't really know...do you get what i mean? lol. I'd say God wouldn't just talk with anyone; throughout the history of religions God has only talked to people important in conveying his message.

As for dream symbolism, you'll just have to go with your own faith. Don't ask yourself "did God put this here?" Instead, ask "why am i dreaming this?" and if you think it's important enough, just trust God is working in some mysterious way. If not, then your subconscious is trying to tell you something; either way, it's important.

Just remember: if you do believe God is talking to you through your dreams (whether it be an omnipotent being or your own mind), you may not like what you hear or see. Often the belief of God is associated with fear -- religions are essentially based upon not going to hell, the most fearful thing, and the fear of hell/disobeying God drives us to be good and follow our religions.

Eh, what i'm trying to say is...hmmm...there's no way of saying this without sounding cliche, but oh well. Trust your heart!