I always thought that meant that as we cannot see the person get born again we can tell by the changes made in the heart. Like when the wind blows you see it in the movement of the trees.
The wind analogy is the answer to Nicodemus’ question of how one can be born again.
3 Jesus answered and said unto him, Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God.
4 Nicodemus saith unto him, How can a man be born when he is old? can he enter the second time into his mother's womb, and be born?
5 Jesus answered, Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.
6 That which is born of the flesh is flesh; and that which is born of the Spirit is spirit.
7 Marvel not that I said unto thee, Ye must be born again.
8 The wind bloweth where it listeth, and thou hearest the sound thereof, but canst not tell whence it cometh, and whither it goeth: so is every one that is born of the Spirit.
9 Nicodemus answered and said unto him, How can these things be?
10 Jesus answered and said unto him, Art thou a master of Israel, and knowest not these things?
Also worth noting in verse 10 Jesus makes clear that Nicodemus should know one must be born of Spirit to enter the kingdom, indicating we can find similar speech in the Old Testament.
See Ezekiel 11, Ezekiel 36, Ezekiel 37...
The new birth is a work of God, by his choice for his glory not a decision or deed of man. To claim one can choose to be born again is to claim the ability to summon God the Spirit and command him to dwell within. One cannot command the Spirit anymore than he can command the wind. The same is true of ones earthly birth, one cannot decide or contribute to his conception or birth.
Man plays no role in either birth, it is the work of God.