It is important to keep in mind that most employers only ask questions that they believe will help them choose the best person for the position regardless of race/color, sex/gender, sexual orientation, religion, disability, age, national origin/citizenship. With that in mind, much of the time if an employer asks an illegal question they are not asking it for the reason it is illegal.
For instance if some asks: "Does your (boyfriend, girlfriend, husband or wife) have a problem with you working long hours since we work 45-50 hours a week?"
There concern is can you work 45-50 hours a week. So they should have asked: "Is there any reason you would not be able to work 45-50 hours a week?"
If you are asked an illegal question you have several options:
Most questions that ask about military discharge, arrest record, disabilities, marital/family status, age, national origin/citizenship, race/ethnic background or any other non-job related attribute are illegal. But keep in mind that some are job related. For example: Many Department of Defense jobs require U.S. Citizenship.