>But how we find out the data content have been changed rather than added?

Here's a likely scenario:

You could set up two additional fields - a datetime field in the lookup table which would keep the time of last save for each individual record, and the time of the last save for any record, in the same place where you keep the primary key counter. This would require some code in the save method (or in the update trigger, whatever you use). The need to update would be checked by comparing the last save time in the local copy versus the last save time in the PK counter table. If needed, you'd update any records where the record's save time was fresher on the server.

YMMV, though, depending on the size of the table, the frequency of changes in it etc.