As koolsid has suggested a timer is one way which could periodically check for a certain parameter you wish to be checked.

Another option is a winsock that could broadcast to all other users that a certain user has made a new record.

And another thing, when just adding a record you could just get the new customer no. the moment you are saving the record so the user will always get the last customer no. if it is being updated. Just make sure it is marked as unique or set as PK.