“They went out from us, but they were not of us; for if they had been of us, they would have continued with us; but they went out that they might be made manifest, that none of them were of us.” – 1 John 2:19
This is how you can tell if someone has been genuinely born again: they will remain. If they do not continue, their leaving reveals their unregenerated heart.
A Verse A Day 