How to understand what Columns Have Changed in Merge Replication
You can use sp_showlineage and sp_showcolv to inspect the lineage and columns that have been changed.  The sp_showcolv results contain a column, colidx, which is the index of the column(s) that were changed for a given version of the row.

If you need to take a more proactive approach, you can setup an auditing scheme.  I've provided an example of how to setup an auditing scheme on my blog:  http://www.sqlrepl.com/sql-server/auditing-changes-in-merge-replication/