“The wind blows where it wishes, and you hear its sound, but you do not know where it comes from or where it goes. So it is with everyone who is born of the Spirit.” (John 3:8)
I think the point being made here is that you can’t see the wind, you can only see its effect. Similarly, you can’t see any physical change in a person born of the Spirit, but you can see the effect of the Spirit at work in someone. We may not understand how the Spirit works, but the effect of the Spirit in the lives of believers is evidence of God at work.