InfiniteFaith,
I must question why you seem to be so desperate to know whether someone else committed detraction. It would be one thing if you were trying to determine what detraction is so that you do not commit it, but in this case, it would seem to me to be a matter for this other person and his confessor, not you. I really see no benefit to you knowing whether or not another person has committed a sin.
Are you still in contact with this person? If you are, and you determine that he has committed detraction, what are you going to do? Rub it is his face that he has sinned? If you are no longer in contact, what are you going to do? Be happy in the knowledge that someone else has committed a sin? Both are uncharitable.