The easiest way would be to use the Property's DataChange event, Thingworx DataChange event provides new and old VTQ so new value, time and quality plus old value, time and quality.
With your use of a DataTable, I'm not quite sure, you may have to retrieve the data, sort it and grab the previous record. Sounds rather complicated to do.
Did you consider using Streams instead of DataTables? Streams provide full access to the content (not like ValueStreams) but work time-indexed so that you can query a stream more easily for your purpose: just specify maxItems to 1 and it'll return the latest entry. This is performance better as well - if you can't follow Pai's proposal which is even better as it doesn't require any data query.