Most Catholics are thrown off by this question and answer hesitantly at best. This leads Fundamentalists to assume that Catholics haven't been born again.
But they are wrong. All baptized Catholics have been born again, so you can answer "yes." Discussion no doubt will follow.
A good way to start is to admit the importance of being born again. Jesus commands it (Jn 3:3,7). To be born again implies a radical change in a person, an event as life-altering as the "first birth" out of your mother's womb. When you are born again, the Holy Spirit makes a change in your soul, cleanses you from sin, and gives you a new nature, planting supernatural love in your heart. Just as you were once born into an earthly family, when you are born again you become part of a spiritual family, with God as the head and all his people, on earth and in heaven, as your brothers and sisters.
Once in agreement on what "born again" means, proceed to the point of disagreement: how we are born again. Probably the Fundamentalist will posit some kind of spiritual experience, a moment of commitment to Jesus, an acceptance of him as "personal Lord and Savior." But Catholics use the biblical means of being born again: baptism.
Jesus told Nicodemus, "No one can enter the kingdom of God without being born of water and Spirit" (Jn 3:5). Paul spoke of God's gift of "the bath of rebirth and renewal of the Holy Spirit" (Ti 3:5). Paul told the Romans that "we who were baptize into Christ Jesus were baptized into his death . . . so that . . . we too might live in the newness of life" (Rom 6:3-4). Water baptism is the physical sign of and instrument for bringing about the spiritual rebirth. In baptism we are regenerated into new life in Christ.
So answer door-to-door missionaries with a proud affirmative--if you have been baptized, you already have been saved the Bible way and in the way the first Christians understood "born again."