Ok, 1 John 4:2
I'll post this link, http://bible.cc/1_john/4-2.htm
as the translations differ, but note that nowhere is this reserved for believers only, or anything; you might simply need to temporarily suspend any disbelief that you can test a spirit, and I say temporarily because I started down this road a skeptic, and I encourage you to ignore this if it does not edify you. Fake it 'til you make it worked fine for me here, and it will for you too. As I recall, irrefutable evidence came for me in less than a week; in the form of silence, when I expected agreement, which I don't expect to convince you, of course.
The question was regarding God speaking to people. Are you saying that the method of knowing whether God has spoken to you or not is to ask the (potential) spirit (who may or may not be God) if Jesus Christ has come in the flesh? If it/he says "yes", then it is definitely God who spoke to you?
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Hebrews 13:23 Know ye that our brother Timothy is set at liberty