There is a simple question, when applied, that is always bulletproof when it comes to identifying truth.

Is this consistent with what God has spoken in His Word?

Truth, is that which corresponds to reality. Clearly, everything in the world is not addressed explicitly in the Bible. Yet everything in the world can be tested by the Bible to know whether or not it is truth.

2 Timothy 3:16-17 All Scripture is inspired by God and is profitable for teaching, for rebuking, for correcting, for training in righteousness, so that the man of God may be complete, equipped for every good work.

Jesus prayed in John 17:17 Sanctify them by the truth; Your word is truth. 

It doesn’t get much clearer than this, and you don’t have to have a seminary education to understand it! Opinions do not ultimately matter. If an opinion contradicts the Word of God, the opinion is wrong. Sincerity does not ultimately matter. If a person is sincere about the wrong things, then their sincerity is wrong.

The rule in summary is this- God has spoken, and He can be trusted. In faith, we believe what God has said. In obedience, we do what God has commanded. If something is contrary to the Word of God, it is always wrong.