So what is this fruit we are meant to inspect? Three answers help. First, fruit is a life. A false teacher can say all the right words while his life preaches something completely different. He may hold the words of life in his mouth without any life being produced in the people around him. What you are watching for is not merely correct speech but a life the gospel has actually shaped.
Second, fruit is teaching, because a man’s teaching proceeds from what he is. Jesus said in Matthew 12 that by your words you will be justified and by your words you will be condemned. What a man says about God is fruit you can inspect. Bad doctrine eventually shows up in the way a man lives and in the way his people live. If someone claims to trust the sovereignty of God yet is consumed by anxiety, notice how belief is showing up in life. How we live reveals what we truly believe.
Third, fruit is what happens to the people who sit under a teacher. John Wesley asked two questions of any teacher: what has this doctrine done to the man himself, and what has it done to the people who follow him? Are the proud becoming humble? Are the worldly becoming heavenly minded? Are the unkind becoming kind? This is not about crowd size or book sales or how you feel driving home. It is about what is actually growing in the lives of those who hear this man week after week.