Not to state the obvious, but "knowing Christians by their fruit" does not mean that everyone who does something "nice" is a Christian. It does not mean that God is pleased with someone just because they give to the poor, attend church regularly, or sing in the choir. It doesn't mean that everyone you think is a good person is saved.
Fruit here refers to the natural attestations of the life one possesses within; it is the fruit of the Spirit. Apple trees naturally produce apples. Orange trees naturally produce oranges. They don't have to try to do this or work at it. In fact, they cannot help it. Even if they tried to produce some other fruit, they do not possess the genetic make-up to do it. Each must and does produce after its own kind.
So it is with the Spirit. To identify those who belong to Him, God has denoted the one thing which cannot be faked: life.
Prior to salvation, we were by "nature" children of God's wrath (Ephesians 2:3). After salvation we can become partakers of the divine "nature" of God (II Peter 1:4).
nature: Greek word physis; it means the nature of things, the force, laws, order of nature; birth, physical origin; the sum of innate properties and powers by which one person differs from others, distinctive native peculiarities, natural characteristicsWhen we speak of our "nature" we are talking about the very essence of who we are...and whose we are. Do we belong to the Lord or do we belong to Satan? How do we know? It is not about what we say or even what we do. It is not even who we think we are. It's about the nature we possess, and each nature has its own fruit.