How does one decide if something is truth or error for the Christian?
The first place one should look is to the scriptures and if these things do not line up with scripture and what the word of God has to say about it then we are to always believe scripture above all other information.
2 Timothy 3:16-17
16 All Scripture
is given by inspiration of God, and
is profitable for doctrine, for reproof, for correction, for instruction in righteousness,
17 that the man of God may be complete, thoroughly equipped for every good work.
2 Peter 1:19-21
19 And so we have the prophetic word confirmed, which you do well to heed as a light that shines in a dark place, until the day dawns and the morning star rises in your hearts;
20 knowing this first, that no prophecy of Scripture is of any private interpretation,
21 for prophecy never came by the will of man, but holy men of God spoke
as they were moved by the Holy Spirit.