A demons whole job is to trick you. There whole existence is to deceive you into breaking the law of God and being with them in all eternity in Hell and Damnation. We, as children of God need to be "wise as serpents and gentle as doves." As such we must ALWAYS be on the lookout for the demons deceptions, for the evil one never sleeps.
When we die, there is the judgement. We are taken before the throne of God and our life is presented to our Lord, our good and our bad. And as we have served in life, so shall we serve after death. For God is always good, and out of His love for us He will give us our hearts desires, so if our heart is evil then hell and the devil shall be our reward. And if our hearts desire has been God and His goodness in our life, Our God, the Creator of the Universe shall be our reward. For eye has not seen nor the mind perceived what God has in store for those that truly love Him.
There are a lot of shows now on ghosts, ghost hunters and "psychic" mediums who speak to those who have passed on. DO NOT LISTEN TO THEM! "No one shall be found among you who makes a son or daughter pass through fire, or who practices divination, or is a soothsayer, or an augur, or a sorcerer, or one who casts spells, or who consults ghosts or spirits, or who seeks oracles from the dead. For who does these things is abhorrent to the Lord..." Deuteronomy 18:10-12.
If we want to see how the demon will try to deceive us, all we have to do is look up "abhorrent to the Lord, abomination to the Lord, in the Bible and we will see what the demon is trying to entice us to do. There are those who will call evil good when they know it will separate us from God, which is their main job. We forget that the evil one knows the law of God better than any of us and he will do anything, ANYTHING to separate us from God.
So when you hear "evp"s that supposedly have spirits speaking or hear a "psychic" medium speak words from the dead we must first ask two questions. 1. Who are you really speaking to? Remember the demons whole job is to DECEIVE YOU into doing what is not right in the sight of God. To that end, demons will LIE to you. So who is speaking through a psychic or in the evp? Most of the time they are demons or demonized humans (humans condemned to hell that seek to wish nothing more than that you be in hell with them.)
How they know.
So what if what the psychic says is correct? What if they have information that no one else could have? Let me explain to you how this works. We are surrounded by minions of the evil one seeking our destruction. They have information about our lives and activities in the past. Demons are intelligent beings and can communicate with one another. So they have all of the past information about loved ones and they can speak through "mediums" and "psychics". So who you are speaking to is a demon, never doubt that. Why would a demon give you information that you find comforting? They don't care how you feel, they just care that you have offended God and they will say and do anything to get you to sin against Him. So if they can tell you how poor Uncle Joe who has passed on liked to play baseball and was left handed - this is information available to the demon that "shadowed" (no pun intended) Uncle Joe in his life. Remember, demons have no obligation to speak the truth but don't think they wont use the truth if it suits their ends - your destruction. There is no verifiable way to identify who or what a spirit is.
The Safest Course
The safest course is to not offend God and to not follow after psychics and ghost hunters and don't watch their programs either. God in His goodness gives us so many tools to push evil from our lives, the Holy Mass, Confession, The Rosary, the list goes on. Seek the company of the Saints and Our Lady instead of spirits because you don't know who are what they may be.