How do we know if we're a Christian?

Is it based upon our works? No.
It is based upon any feelings we have inside us? No.

1 John 4.13-15
By this we know that we abide in him and he in us, because he has given us of his Spirit. And we have seen and testify that the Father has sent his Son to be the Savior of the world. Whoever confesses that Jesus is the Son of God, God abides in him, and he in God.

We know we are Christians because his Spirit is in us.

And how do we know if his Spirit is in us?

If we confess Jesus as Lord.

Therefore (from my sermon on 1 John 4.13-21):
All Christians have the Holy Spirit.
The Spirit of God has been given to us as a seal of our salvation.
His presence in us convinces us that Jesus is the Son of God,
and was sent by the Father as the saviour of the world.
His presence in us proves that we are saved,
and gives us the confidence that we will not be punished by God.
His presence in us enables us to love God and each other.

This post was inspired by Christopher's comment at BHT.