This is really an excellent question and was one of the first I asked myself in my quest for the "truth" of Yahweh.
How do you know if what you hear is the truth or a deception?
1. Can the statement be backed unequivocally by scripture?
If it can't be found in the bible, it is merely opinion and is the recognized outcome of environment and culture. If Yahweh says it, only then it is considered truth.
2. What is the purpose of the statement?
This question may seem inconsequential, but I believe it to be important. All statements, even statements of biblical truth, are said with a motive in mind. If you are hearing it in the vein of helpfulness for a query you have posed, then that is one thing, but I hear more and more arguments break out merely because someone wants to be superior. So many people have to be "right" that it ceases to be an instructional discourse, and slides rapidly into a childish "I know more than you do" spat.
If you are receiving an instructional or inspirational message which helps you learn or become a little more emotionally balanced, then it is to be considered, otherwise, it shouldn't.
3. Is it all that important?
I don’t mean that truth isn’t important; truth is all that is important. What I am referring to is the message being conveyed; is it really important enough to worry about? Is it a life changing thought? Then this needs to be clarified and the truth sought out earnestly. Is it merely a small detail that has no major impact on your way of life? Then, maybe, this is an instance where it isn’t necessary to debate, but to tuck away in the back of your mind as something to discuss with others. This is where fellowship comes in to play. Fellowshipping with others is necessary to bounce ideas and thoughts to and fro and to come up with a logical and sensible meaning.
When you pick up your Bible to learn about God, then you are practicing truth. Like anything else, the more you practice at it, the better you are. Soon, the truth will shine out brightly and the deceptions will be obvious and flat.
Pray incessantly, study diligently, and love each other. Then you will know the truth when you hear it.