Gemoro Shabbos 12b:
Rabbi Yishmoel ben Elisha inclined the lamp on Shabbos against the
halocho and wrote in his record book that when the Temple would be
rebuilt, he would bring a fat korban chatos to atone for the sin.
From this we see that it is important to atone for a sin done in the past even at some distance in time.
Gemoro Shabbos 68b
A child who was brought up by non-Jews and never knew anything of
Judaism, when he later comes into the community and learns, will have
to bring one korban chatos for all the infringements of the Shabbos
laws that he committed until that point.
Again we see that it is important to atone for a sin done in the past when one gets to know about it - even at some distance in time.
Therefore, I deduce that it is important to determine if one has sinned in the past in order to get atonement for it.