SELECT PrimaryKeyColumn, TRY_CONVERT(xml, VarcharColumn) AS XMLValue, VarcharColumn FROM YourTable WHERE TRY_CONVERT(xml, VarcharColumn) IS NULL AND VarcharColumn IS NOT NULL;
This should identify the problem rows.
Thanks- that's exactly what I ended up doing. I'll use the resulting NULLs to pull the problem rows into an error table to be addressed later.