God's voice will do one of three things: it will either encourage you, convict you, or enlighten you.
satan's voice will do the opposite.
And of course, you can't forget your OWN voice in all this mix. I think sometimes we dream up all sorts of things and say "God told me this" when in fact, it's nothing more than our own minds.
A bit more in-depth explanation:
1) encouragement vs. discouragement: A message from God will give you hope in the face of adversity (i.e. if you are depressed, and suddenly a friend calls you out of the blue and tells you he believes God has an awesome future for you**.... or if the small voice within you is telling you everything will be okay...that is likely from God. **i don't limit God's "messages" to only the mind.... I believe He can and does use circumstances to speak to us).
satan, on the other hand, will always try and plant thoughts such as "i'm not good enough," "i'm ugly," "i have no future".... etc.... in your head. these thoughts are ALWAYS from the evil one.
and of course our own imaginations.... i'd attribute that to when things get a bit too specific. say, "God told me i will marry so-and-so..." or "I'm totally sure I'll win this competition because God told me I will." Well, 99.9% of those are wrong.... it's just your own overactive heart telling you exactly what you want to hear. Of course, you may end up marrying that person or winning that competition... I just see no reason for God to habitually reveal these things supernaturally.
2) Conviction vs. condemnation/dismissing sinfulness: If you, say, steal something, and a voice inside you is telling you that you messed up, and you have to stop stealing.... that'd be from God. However, if the voice harps on you for being such a stupid, evil, unlovable person, that's satan talking. Sins are forgiven through Christ....we are called to accept His sacrifice, stand humbly before Him, and repent of our sins. We know we are sinners, but we are forgiven and walking a new path in His ways, and any more condemnation is unjustified.
There is, of course, satan's way of erring on the other side. "Oh, it's okay that you stole those clothes. You can keep doing it... it doesn't matter. God forgives you anyway... nobody's perfect." This is minimizing God's glory and Christ's sacrifice. We are not called to keep sinning just because grace is given.
I don't think satan has to work particularly hard on this subject.... with our own voices, we tend to err on one side or the other. the key to hearing God's voice with this is, if it elevates Christ, it's from Him. if it doesn't, it isn't from Him.
3) Wisdom vs. confusion: if you're learning more about life, about Him, and about who He made you to be.....and if you desire to keep seeking to learn more.... that is from God. However, satan likes to twist this and use it to suit his own purposes.... he'll either try to get you to believe that knowledge either makes you better than everyone else, or isn't useful at all. And of course, satan will try to get you to believe false information (to test this, check it in line with the Word) or get you confused.