You have, I believe, two choices. Simple(r) one. If you know adding a column was last DDL applied to that table, check
SELECT TO_CHAR(LAST_DDL_TIME,'MM/DD/YYYY HH:MI:SS AM') FROM USER_OBJECTS WHERE OBJECT_NAME = 'TBL'
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Otherwise, use DDL event trigger. However, it is not that trivial as Barkovsky and strm sound. As far as I know system event ora_is_alter_column is set if column is being added or modified, therefore you most likely will need to parse result of ora_sql_txt event - the triggering statement - to tell for sure.
SY.