Because thou hast kept the word of my patience, I also will keep thee from the hour of temptation, which shall come upon all the world, to try them that dwell upon the earth. Rev. 3:10.
You cannot judge truth on the basis of the presence of undeniable power, or by miracles, or healings. You cannot judge truth on the basis of warmth and fellowship and love, or even changed lives. So on what can you judge truth? How can anyone come up with conclusive proof?
Well, it is certain that we can know truth. We have been promised, "Those who are earnestly seeking a knowledge of the truth and are striving to purify their souls through obedience, thus doing what they can to prepare for the conflict, will find, in the God of truth, a sure defense. 'Because thou hast kept the word of my patience, I also will keep thee',...is the Saviour's promise. He would sooner send every angel out of heaven to protect His people than leave one soul that trusts in Him to be overcome by Satan."--The Great Controversy, p. 560. We have big forces on our side that we can count on.
The first way to test the truth of supernatural manifestations is by the Word of God. Even people who apparently love one another and Jesus and the Bible, but who do not keep the commandments of God, cannot necessarily be depended upon to be from God. "If they speak not according to this word, it is because there is no light in them" (Isa. 8:20). If a group is experiencing wonderful manifestations, but at the same time is teaching that you don't have to obey all of God's law, there's a big clue right there. Although it may be true that there are sincere individuals that God is leading to Himself in every group, this fact does not prove that we should abandon the truths that have been revealed to us as a church and go to join them. They may be on their way up, but if we join them, we may pass them on our way down.
An equally important safeguard against deception is to have the righteousness of Christ in the life, to be in vital relationship with Him. Even a head knowledge of Scripture will prove insufficient to take us through the times ahead. But as we continue the fellowship with Christ, solidly based on His Word, we will be secure against all the wiles of the enemy.