Various federal, state, and local laws regulate the questions a prospective employer can ask you. Questions must relate to the job for which you are applying!
You have options if you are asked an illegal question
National Origin / Citizenship
Employers are allowed to ask if you are authorized to work in the US. They are not allowed to ask if you are a US citizen, were born in the US, or your native language.
Employers are allowed to ask if you are over the age of 18. They are not allowed to ask how old you are, your birthdate, or the year your graduated from college if it isn't on your résumé.
Marital / Family Status
Employers need to know if you are able to perform the requirements of the job - including working overtime, relocating, or traveling if necessary. These questions should be asked directly. They are not allowed to ask about your marital status, children, or daycare arrangements so they can make assumptions if you are able to meet these requirements.
The social organizations or clubs you belong to are personal. The employer may aks if your membership in any organization might be relevant to your ability to perform the job.
Your height and weight is personal. The employer's right is restricted to knowing if you can fulfill the physical requirements of the position (e.g., lifting a 50-pound weight).
Employers are not allowed to ask if you are disabled or for your, or your family's, medical history. They may ask if you are able to perform the essential functions of the job and even request you demonstrate job-related functions. Once hired, companies are allowed to have you undergo a physical examination. Results are confidential with the exception that medical/safety personnel may be notified of conditions which could require medical treatment and supervisors regarding the needed accommodations.
Employers are not allowed to ask if you have been arrested but may ask if you been convicted of a crime that is resonably related to the performance of the job.
Employers are not allowed to ask if you were honorably discharged from the military. They must limit their questions to the branch of service or training and education received during your time in the military.