I answer as one with experience and who knows by God's confirmation.
There are many spirits, but only One is the spirit of God.
I was "caught up to the third heaven and heard words unlawful to utter." Which came before I even knew there was a god or God. But He who guides the steps of men, who makes manifest all things for His own purpose, brought me to the brink and to calling out to Him for no other reason than as a last resort. And He answered.
Which, if I had not called Him by name, or had He not arranged for me to have time and opportunity to read His word shortly thereafter--from cover to cover--wherein He then confirmed my encounter with Him to be God, but also that His word is true, as the Author of both...I could have, and would have easily passed off the whole experience as foolishness. So, He Himself caught me up in the spirit, and also confirmed just who He was without question, but with confirmations beyond number connecting all things, even everything leading up to and of my whole life.
But God has also sent evil men and armies to turn nations, promised strong delusion, and allowed false Priests among Israel and false teachers within the Church. All of which can only be truly known by the Spirit. Therefore, do not expect that there should be a way of knowing otherwise, or a proof which the ungodly (atheists) demand, but rather that God has set before us all life and death, blessing and cursing, that we each must choose. As it is written, this is the valley of decision.