Well, first of all it is against the law to ask about your legal status during an job interview. When filling out the I-9 all you have to do is check the last box on the very first part of the form where you fill out.

*An alien authorized to work (Alien # or Admission #)____________
until (expiration date, if applicable - month/day/year)____________

So, basically that is the only time where you have to tell other people about your legal status.