Hi,
Use table maintenance events.
You can build the validations as you required in TMG.
For additional information refer:
EVENTS IN TABLE MAINTENANCE - ABAP 开发 (ABAP Development) - SCN Wiki
http://wiki.scn.sap.com/wiki/display/ABAP/TABLE+MAINTENANCE+GENERATOR+and+ITS+EVENTS
Extended Table Maintenance Events - ABAP Development - SCN Wiki
Regards,
Chandra