Then the answer is "through the mind of Christ given us".
Does the Spirit reveal all truths to us instantly? No. There is progressive revelation, as documented in Scripture, and as proven by a Beleiver's growth in Christ and progress toward maturity.
As we are still in bodies of flesh, much is not revealed to us...
and yet many form opinions about topics that are not revealed to mankind.
When people call "heresy" on these topics, I call them out on it.
But the basis of Truth, is what is revealed in Scripture, and unveiled by the Holy Spirit dwelling within the Believer.
The Spirit does not contradict itself, so in debates over theologies,
We must discern what is of Spirit, and what is of flesh,
What is useful, and what is philosophical,
What is concrete, and what is conjecture.
Sometimes, we have to just admit that God has not yet revealed certain things.
Other times, we have to refute those who twist the Word.
And yet more often, we have to edifying those who are misinformed of the Word, or who have misplaced zeal.