The concept of original sin is to eat (mentally consume) the knowledge of good & evil. Genesis 2.15 explains:
The Lord God took the man and put him in the Garden of Eden to work it
and take care of it. And the Lord God commanded the man, “You are free
to eat from any tree in the garden; but you must not eat from the tree
of the knowledge of good and evil, for when you eat from it you will
certainly die.”
We know Adam & Eve did not physically die when they ate the fruit. Their "death" was a spiritual death. Adam, Eve, Cain & Abel began to have arguments & conflicts over their differing opinions of what was good vs evil, which at its extreme lead to Cain murdering Abel.
Therefore original sin is provable because every act of sin is based on a view of something being 'good' or 'evil'. A person steals or commits adultery because they believe the object of their lust is 'good' or 'desirable'. A person murders because they believe another person is 'evil'.
The flesh itself does not sin. Only the mind sins. James 1.13 says:
When tempted, no one should say, “God is tempting me.” For God cannot
be tempted by evil, nor does he tempt anyone; but each person is
tempted when they are dragged away by their own evil desire and
enticed. Then, after desire has conceived, it gives birth to sin; and
sin, when it is full-grown, gives birth to death.
While not Biblical, the The Apocryphon of James provides a good explanation:
For without the soul, the body does not sin, just as the soul is not
saved without the spirit. But if the soul is saved (when it is)
without evil, and the spirit is also saved, then the body becomes free
from sin. For it is the spirit that raises the soul, but the body that
kills it; that is, it is it (the soul) which kills itself.