There are many spirits. There is only one source of good Spirit - God. There is only one source of evil - the Devil, the serpent, the one who masquerades as an 'Angel of Light'.
There are many NT references to God's Spirit 'speaking' to people - i.e. God is revealing truth into the spirit in men by His spirit of truth.
Many have claimed of late to have heard the 'word of God' and He told them that Mr Trump would be returned to power. These 'Christians' claimed to have the truth and was specially revealed to them to proclaim to the people.
Sadly, there is much disappointment that they were totally wrong. Clearly God was not wrong. They were claiming falsely that God had 'spoken to them in the spirit'.
How might we test this spiritual 'truth'? Or any other 'spiritual truth'?
The primary way is to look to God's word which has been largely preserved to reveal God's personal being and manner, His son Jesus, His plan for creation and the salvation of it.
Things which are not written there are to be questioned regarding their veracity. No matter who wrote them.
God has allowed corruption to interfere with practically every thing He has done on the earth from the garden onwards. We may understand from this that God intended man to experience the horror of sin as part of His gracious salvation - depicted in many respects by the message about Jesus being foreknown and 'slain from the foundation of the world'. Rev 13:8
He was known before the foundation of the world, but was revealed in the last times for your sake. 1 Pet 1:20
His word has also suffered corruption and interference over the centuries. What we have today contains error and bias not intended by the writers or the God who inspired them. Yet, thank God, the overall narrative remains intact and a source for great encouragement and truth of things that matter.
The spirits that tell things not of the word, or of things that extrapolate too far from the word, or things that conflict with the word are not of God - He is not the author of confusion or contradiction.
For ex. The devil is the great deceiver. Those who deceive and misdirect from plain truth are not of God. We might ask what the following passage means.
For many deceivers have gone out into the world, refusing to confess the coming of Jesus Christ in the flesh. Any such person is the deceiver and the antichrist. 2 John 1:7
If Jesus is God, is he really flesh? Is God flesh? No, He is spirit. There is no plain teaching that Jesus is God anywhere in the bible. Many have used bits of scripture to suggest otherwise and the questions and thoughts on this matter fill the pages of this site and many others.
The truth of the word of God, the bible (even with the additions that form a bias to another gospel, another God) is clear about who Jesus is. If he is somehow God - he is not flesh but God, he is not made like one of us!
it was necessary for him to be made in every respect like us, his brothers and sisters, so that he could be our merciful and faithful High Priest before God. Heb 2:17
God is not flesh. Those who proclaim some pre-existing Jesus - who is NOT like us in every respect, speak not the spirit of truth, but some other spirit is guiding them. We are warned to recognise and then avoid such spirits.
How Jesus can be God and also High Priest before God is a mystery not of the text.
Each of us has to honestly enquire of the truth on such matters and, hopefully with God's grace, it will be known. We will have 'tested the spirits' and this not through careful study of the most prominent authors - but by earnest prayer and a humble adherence to the written truth already provided.
Perhaps many, or most, over the ages have not had opportunity to know the whole truth and have died being somewhat deceived. This matters not for salvation - God is gracious and planned through the resurrections to give each their opportunity. Let us not judge one another - this is Jesus' job.
God...who desires all people to be saved and to come to the knowledge of the truth.
He is bigger than sin, bigger than deception, bigger than evil and will see His beautiful creation redeemed to the glory He intended through His son Jesus.
The god (Satan) of this age has blinded the minds of unbelievers so they cannot see the light of the gospel of the glory of Christ, who is the image of God.
Through prayer and genuine seeking of truth from God's word, God willing it will be obtained - perhaps in this age, certainly in the next!