how to make a private attributes in STI?

people table like this

id int not null auto_increment,
type varchar not null
/* common attributes */

name varchar not null,
email varchar not null,
/* attributes for type=Customer */

balance decimal(10,2),
/* attributes for type=Employee */

my silly question is though the autoer note which attribute belong who,
but in the real world, real code, how can I know which is private
attribute for customer or employee?

any thought here?