How can people tell the difference between a prophet and someone who hears voices in their head?

Same thing.

Time ...

I don't know, but some people are more "tuned"  to what we call the paranormal. It's just the way they are. 
If you believe in reincarnation, they probably have remaining fewer cycles to go through than the rest of us. But this is only my personal opinion. This post was edited by Benedict Arnold at October 18, 2016 8:38 PM MDT

Mostly by what is foretold and what actually happens versus how important  that message happens to be.  

They can't... It will all be up to their believes... Do they want to believe the "prophet" or not.

Well it's pretty easy today as the Bible is complete and Armageddon hasn't happened yet so no new scrolls yet. You can see a distinction between one serving God and one not by their fruits. Prophesying was to be done away with after the Bible was complete. Gotta remember that there is a difference between trying to understand previous prophecy's as recorded in the Bible and trying to understand previously recorded prophecy's. At the bottom is one such prophecy that to me isn't too deep for the average person to understand.
In a nutshell, anyone claiming to have a new prophecy today is not for real. (Again, trying to understand a previously recorded prophecy is not the same as saying you have an inspired prophecy from Jehovah.)


(Malachi 3:18) And you will again see the distinction between a righteous person and a wicked person, between one serving God and one not serving him.”
(1 Corinthians 13:8-10) Love never fails. But if there are gifts of prophecy, they will be done away with; if there are tongues, they will cease; if there is knowledge, it will be done away with. 9 For we have partial knowledge and we prophesy partially, 10 but when what is complete comes, what is partial will be done away with.


http://answergang.ning.com/forum/topics/are-you-familiar-with-the-prophecy-about-the-timing-of-the

Interesting question worth exploring.

1- The phrase “hears voices in their head” is almost always used metaphorically, to imply that the person who does that is mentally ill. However, the fact is that everyone hears voices in their head. If a person is brain dead they can't hear, even though their ears are free of defects.

2- We usually hear voices of people we can see, but it is also quite normal to hear voices of people we can't see. This could be voices of people in the background, over a PA system, on the phone or on an audio clip.

3- Whatever we hear as humans has to be within the frequencies audible to humans. Certain animals can hear sounds which to us humans "don't exist" So those animals, or any human with a bit of that ability, are on the face of it “hearing voices [sounds] in their head”

4- People who talk in our dreams may not actually exist, but what they said exists, if we remember it. Mostly we "hear" conversations between others, but sometimes the person in the dream is talking to us. Whether that’s happening all the time but we’re not conscious of it or it only happens only when we’re asleep and switches off as soon as we wake up, I don’t know. Either way it is quite normal.

5- Perhaps in schizophrenia, which is clearly an instance of the metaphorical “hears voices in their head”, it's the same or similar process as above, but it’s taking place in the conscious part of the unfortunate person’s brain and while they are wide awake. The persons are in a way “living their dream” but, painfully for us observers, it’s a literal “living their dream” rather than a metaphorical one.

6- Now what if the person who “hears voices in their head” is told to strike the sea with the rod in his hand and the sea will part, and he does so, and the sea does part? What if he’s told that everything subjects itself to laws and the source of these laws should be called God and our observations confirm the first part and there is no real reason why we can’t call the source God?  What if he’s given a set of laws for people to live by with the claim that it will benefit them in this life and after death, and we can see the benefits of the laws in this life and have nothing to disprove the claim about after death? Is such a person a prophet even though he’s someone who has "heard voices in their head"?

An official prophet of the Lord always had God's support and approval. 

'Prophets' quite often have visions rather than hear voices like Abe Lincoln foreseeing his own death about ten days before he was assassinated or Joan d'Arc having visions of leading the French army to defeat the invading English army and kick them out of French territory. 

Very simple are the accurate. If they say this will happen and the opposite happens.... you can say they are false prophets.