Checking modification date on a row, in ms sql 2008

You cannot have the information when the information is not available.
Since you cannot alter the original table, one way I can think of is to create a new table that has only 2 fields - an id field and a datetime field. The id field will link to the primary key of the original table. Whenever a row is inserted, updated, or deleted in the orginal table, it should also insert/update the corresponding id row in the new table with the current datetime value. You may use trigger to automate this process. In this way, you can keep track of any changes made to any row in the original table.