That is such a good question. I decided to ask that same question in a Facebook group. So many people in this particular group have had visual and audible experiences with Jesus and also demonic spirit manifestations. So I decided to ask them how they knew which spirit was which. Certainly demonic manifestations can be obvious, but not always.
We do know that Satan appears as an angel of light. He is not (the light) and he is not (the angel of light), but he appears (as) an angel of light.
So we can expect Satan to appear as light, to appear as a good thing, and not as something we can easily recognize. It is our pride that makes us feel secure and believe we would (just know) because we have such discerning abilities.
Satan deceives and he is the master of deception. If he reveals himself as darkness, where is the deception? But Satan appears as a light, he gives us a false confidence, a false comfort, a mimicking of the holy things of God. He convinces people he is God and that they are on the right path. Many will even perform miracles in "god's name" but Jesus will say in that day, "Depart from me; I never knew you you." Matthew 7:23
The Bible tells us to test every spirit to see where it is from. We are admonished to NOT believe every spirit.
So, how can we know the spirit that manifests itself to us is from God?
Is it the really good and comforting feeling we get from this spirit? The feeling that says "this is all good, listen to this person or me for that matter? That "knowing" feeling, where you "just know."
Is it because we feel special and loved possibly for the first time? Is it because we see a glow or aura that we believe this is a sign or confirmation from God?
So many times too we hear people making statements such as, "I prayed about this and this is what I think or how I am going to respond" as if to imply that their actions or words are from God when even a small amount of understanding of scripture would make it obvious that God was not in their actions or words. If we will be judged for every idle word how much will we be judged for implying we are speaking or acting for God when we are actually being self serving and acting from our carnal nature?
We may think that good and evil seem simple enough to discern, but Satan is the deceiver. Satan has a veil over the mind of unbelievers; it is only through Christ this veil is removed and there are many who believe they are saved yet the veil (of Satan) is coving their eyes and ears. They believed a lie from the deceiver who most likely appeared "as an angel of light."
The Wide Gate
It is an easy way. It doesn't require any repentance or confession of sin, and certainly no turning from sin. You don't even have to see or pass by the blood of Jesus.
The Narrow Gate
"Enter through the narrow gate; for the gate is wide and the way is broad that leads to destruction, and there are many who enter through it. 14"For the gate is small and the way is narrow that leads to life, and there are few who find it. Matthew 7:13-14
We need to know the word of God and recognize ANY no matter how small or seemingly inconsequential we may think it is... If it deviates from scripture, it is a lie and it is major! Back away from that spirit. It is NOT from God! It is the tiny lie (twisting of truth) we need to look out for, not the obvious lie.
Let's look at what the word of God says regarding these spirits:
Test all things; hold fast what is good.
1 Thessalonians 5:21
Two or three prophets should speak, and the others should weigh carefully what is said. 1 Corinthians 14:29
Dear friends, do not believe every spirit, but test the spirits to see whether they are from God, because many false prophets have gone out into the world. 1 John 4:1
And when they say to you, “Seek those who are mediums and wizards, who whisper and mutter,” should not a people seek their God? Should they seek the dead on behalf of the living? 20 To the law and to the testimony! If they do not speak according to this word, it is because there is no light in them. Isaiah 8:19-20
Beware of false prophets, which come to you in sheep's clothing, …
17 These were more fair-minded than those in Thessalonica, in that they received the word with all readiness, and searched the Scriptures daily to find out whether these things were so. Acts 17:11
13 For such are false apostles, deceitful workers, transforming themselves into apostles of Christ. 14 And no wonder! For Satan himself transforms himself into an angel of light. 15 Therefore it is no great thing if his ministers also transform themselves into ministers of righteousness, whose end will be according to their works.
2 Corinthians 11:13-15
"You will know them by their fruits. Grapes are not gathered from thorn bushes nor figs from thistles, are they? 17"So every good tree bears good fruit, but the bad tree bears bad fruit.… Matthew 7:16-17
The Works of the Flesh
Galatians 5:19 Now the works of the flesh are evident, which are: adultery, fornication, uncleanness, lewdness, 20 idolatry, sorcery, hatred, contentions, jealousies, outbursts of wrath, selfish ambitions, dissensions, heresies, 21 envy, murders,[d] drunkenness, revelries, and the like; of which I tell you beforehand, just as I also told you in time past, that those who practice such things will not inherit the kingdom of God.
The Fruit of the Spirit
22 But the fruit of the Spirit is love, joy, peace, longsuffering, kindness, goodness, faithfulness, 23 gentleness, self-control. Against such there is no law. 24 And those who are Christ’s have crucified the flesh with its passions and desires. 25 If we live in the Spirit, let us also walk in the Spirit. 26 Let us not become conceited, provoking one another, envying one another.