Summary: We need to get as familiar as possible with the real God of the Bible. Then we'll find it easy to recognize the counterfeits.
Ephesians 4:14 Then we will no longer be infants, tossed back and forth by the waves, and blown here and there by every wind of teaching and by the cunning and craftiness of people in their deceitful scheming. 15 Instead, speaking the truth in love, we will grow to become in every respect the mature body of him who is the head, that is, Christ. 16 From him the whole body, joined and held together by every supporting ligament, grows and builds itself up in love, as each part does its work.
Maturity & Understanding, Love & Work
Maturity, and understanding what teaching is biblical, and what isn't, go together. In my own very unscientific experience, people who seem to me to be deeply mature spiritually also tend to have a certain perspective concerning Scripture. They don't talk much about interpretation or sophisticated theological arguments. Instead, they know, believe, and try to live according to what the Bible teaches. A long perspective helps in this. If you spend years going through the Bible from Genesis to Revelation, you'll begin to see trends and repeated themes. You'll begin to see God as both loving and just, true and gracious, unimaginably distant (and different) and impossibly intimate. He is a bundle of paradoxes. Love and justice don't seem to go together, because justice demands punishment. Truth and grace seem to be in conflict because truth is austere, but grace is compassionate. God is so foreign to us, yet He knows the number of hairs on our heads (between 100,000-300,000 hairs. I'm on the lower end of that span).
You'll see God as Father, Son and Holy Spirit-united yet distinct. You'll think of the Father as your Provider & Discipliner, Jesus as your own Redeemer & Friend, and the Spirit as your Guide & Helper. You'll begin to have a global perspective concerning human history. We are a small link in a chain that unites humanity's earliest beginnings with our ultimate end. You begin looking forward to the reward of the righteous and the judgment of evil. Death loses its numinous fright and life becomes merely the means by which God works out His purposes among people. You'll look to God for your advice and choices, and honor His direction over that of friends, or popular opinion. You begin to respect the whole Bible, Genesis to Revelation, and to be very skeptical of claims to truth that don't match its teaching. I could make a list of strange teaching from non-Christian organizations that claim to be Christian, but there's no need.
In US banks they have an interesting way of training tellers to recognize counterfeit $20 bills. They have them handle about 20,000 genuine $20 bills, then sneak in one counterfeit. Most trainees recognize the fake immediately. We need to get as familiar as possible with the real God of the Bible. Then we'll find it easy to recognize the counterfeits.
I want to be truly mature. Teach me both truth and love. Make me stable, unmoved by the winds of teachings that don't match Yours, and temptations that draw me away from Your will for me.