​How can a person tell when a thought is God’s Voice or when it is Satan’s lies?

One key indicator is that Satan’s lies are bent on attacking my value and destroying my desire to come to God.
Phrases like, “You’ve messed up again, God won’t want to see you!” and “You are such a failure as a Christian!” may sound like rational thoughts when considering the evidence, but they are in direct opposition to the truth that God’s voice always invites me to come to Him, regardless of where I am.
“Come now, let us reason together,” says the Lord, “Though your sins are as scarlet, they will be as white as snow; though they are red like crimson, they will be like wool.” (Isaiah 1:18) 

If attacking my value does not keep me away from God, Satan will use the more subtle tactic of deceiving me about my true condition and need for God's changing grace.
Thoughts like, “I’m doing so well, praying and reading God’s Word.” “Everything is going well, I must be doing something right!” can mask pride and give a false security that equally destroys a desire to come to God. A person without an inner thirst will not come to God. A longing, a desire for the inner empty to be filled, a dissatisfaction with plastic facades and empty motions--that place of neediness and thirst for righteousness is exactly what delights the Father:
"Come, all you who are thirsty, come to the waters; and you who have no money, come, buy and eat! Come, buy wine and milk without money and without cost." (Isaiah 55:1)

Jesus made it very specific that the thirsty one, the needy one, the distressed one crying out, is crying out to Him--and the one who comes to Him will be satisfied, not just one time, but continually. The invitation is open.
“Jesus stood and said in a loud voice, ‘Let anyone who is thirsty come to me and drink. Whoever believes in me, as Scripture has said, rivers of living water will flow from within them.’”
​(John 7:37-38)

You know what the scary part was? The people who studied the Scriptures were the ones who wouldn’t come to Jesus. The reason? Although the motions made it look like they were seeking God, they weren’t thirsty. They didn’t feel a driving need that made them want to be changed. Their knowledge of Scripture kept them insulated from seeing or feeling their true condition—that they were not filled with compassion and love for others, they were concerned only for their own wellbeing.
“You study the Scriptures diligently because you think that in them you have eternal life. These are the very Scriptures that testify about me, yet you refuse to come to me to have life.”
(John 5:39)

Whether we hear the lies that we have messed up too much and God doesn't want us to come, or we are subtly deceived with feeling no need to come, the invitation remains the same to the end:
“Let the one who is thirsty come; and let the one who wishes take the free gift of the water of life.” (Revelation 22:17)

 But then in II Samuel 11, there is the Big Black Splotch--David's adultery with Bathsheba.  And when the news of Bathsheba's pregnancy gets to him, he tries to rub the messy splotch with panicked plotting to cover it up, finally sending an innocent man to his death in battle.  
David has made a mess on the page of his life--and he did it not in youthful ignorance, but when he was in his stride following God.  Somehow, that makes the dark spot look messier and splotchier. 
David's repentance and broken heart are evident.  And when Solomon (the second son by Bathsheba) is born, "the Lord loved him; and because the Lord loved him, he sent word through Nathan the prophet to name him Jedidiah." (12:25)

But wait.  Back up on the page.  II Samuel 7--the passionate heart part of the story, before the splotch: "When your days are over and you rest with your fathers, I will raise up your offspring to succeed you...I will be His Father and he will be my son.  When he does wrong, I will punish him with the rod of men, but my love will never be taken from him as i took it away from Saul."    I have always filled in Solomon's name in my mind as I have read that passage, because I know the end of the story: but DAVID doesn't know the end of the story!  He doesn't know about the dark splotch and his failures, or that Solomon is going to come from the messy part of his life.  
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God wasn't caught off-guard, scrambling for a way to incorporate Solomon into His plan so that he could make David feel that he was forgiven.  God already had the dark splotch as an integral part of the Big Picture HE was working--before the splotch was ever on the page. 

I've noticed that believers often feel more forgiven for sins they committed before trusting in Jesus.  There is a false perception that Jesus covers the sins up to repentance, but that I am responsible to keep the page clean from then on.  (Note: this isn't meant to imply that we aren't called to holy living!)  
I've also noticed that believers who have many dark splotches on the life-page before coming to Jesus, somehow feel that God's purpose for them starts at the point of repentance, and the past is an unfortunate mess that He has to step over to get to His Real Plan for them.  
 
Our disappointment with ourselves and regrets from the past not only fuel the accusations of the Enemy to discourage us, they also keep us stuck thinking that an irretrievable clean page means it is a wasted part of the story.    

In I Samuel 13 Saul has 2,000 scared Israelite soldiers waiting to face the formidable army of the Philistines, who had amassed an army with 30,000 thousand chariots and 6,000 horses and innumerable foot soldiers.  He is scared and his army is scared.  Over the next seven days, Saul's Israelite soldiers begin to desert by the THOUSANDS (I Samuel 13:2,15).  What is Saul's hold up? Only that Samuel hasn't arrived to offer the sacrifice to secure the blessing of God for the battle. Based on what Saul SAW in front of him, he concludes that he must do SOMETHING to salvage his dwindling army. 

He decides to offer the sacrifice himself in order to rally his troops and secure the blessing.  His mindset revealed a "lucky charm" approach to God very similar to when the Israelites had marched God's Ark into battle, thinking that would assure them victory over the more powerful Philistines. Now, not so many years later, Saul's trust was also misplaced in a ritual of invoking blessing because his heart was far from God. 

Even as Saul is having the animals prepared for sacrifice, Samuel is walking the last mile or two and arrives as the fire is still smoldering on the altar.  Very similar to God's tone to Adam in the Garden of Eden when He calls His first leader to account, He now addresses His first appointed king over His people through the voice of Samuel:
"WHAT HAVE YOU DONE?"
And like the first leader in the Garden, Saul's answer shows a lack of sorrow over his disobedience to God and disregard for His ways. Notice the subjects of Saul's answer:
"THE PEOPLE...were scattering."
"YOU...didn't come in time."
"THE PHILISTINES...were assembling."

What had seemed so right and logical to Saul based on what he saw in front of him was actually the most foolish decision he had made in his life. When God told Saul that the kingdom would now be taken from him because of his disobedience and given to a "man after His own heart" it was not a "one strike and you are out" situation.  Rather, it was the commentary on an anointed and appointed leader who had an unrepentant heart toward his own sin.