That is inherently James' reflection upon what Abraham had done in life.
But how would it look to Abraham, while he was alive?
I believe my faith is being made complete by what I am doing and what I will do.
James has the benefit of a perfect hindsight view of Abraham's life. But while Abraham was alive, he could only know what he had done, what he was doing and what he hoped to do.
And how could he know his faith would in sum at the end of his life have been made complete by what he had done? He could hope, but how could he know?
Born again is an idea that shows up repeatedly in the New Testament. We die with Christ, and are risen with him. "...if anyone is in Christ, he is a new creation."
Justification is always in the past tense - at least, everywhere I can think of. No where does it say 'Have faith, do good deeds, and see what happens'.
As a practical matter, how can you tell for yourself, or others?
For myself, I have the Spirit within me..."5 For you did not receive the spirit of slavery to fall back into fear, but you have received the Spirit of adoption as sons, by whom we cry, "Abba! Father!" 16 The Spirit himself bears witness with our spirit that we are children of God, 17 and if children, then heirs—heirs of God and fellow heirs with Christ, provided we suffer with him in order that we may also be glorified with him."
With others, there is no certain way to tell. I had a roommate that I assumed was Christian, but one day he walked in and announced he had become a Buddhist. I girl I knew in college SEEMED like a Christian - but one evening, she was talking with a friend she had known for 10 years, and burst into tear. "I've never become a Christian", she cried.
"So, what are you waiting for?", replied Tip.
Meanwhile, I've seen non-Christians do many good things while denying God exists.
After all, even St Paul wrote: "12 For what have I to do with judging outsiders? Is it not those inside the church whom you are to judge? 13 God judges those outside. "Purge the evil person from among you."