You should avoid asking questions that could be viewed as discriminatory or that infringe on a candidate’s privacy. Essentially, you want to avoid any questions that are not directly related to the candidate’s ability to perform the job.
These would include questions about race, national origin, citizenship status, religious affiliation, disabilities, pregnancy (or desire to become pregnant in the future), sexual orientation or gender identity, past illnesses (including use of sick leave or filing of workers’ comp claims), age, genetic information, or military service. You should also consider state laws that might apply, like prohibitions on asking for salary history or requesting that a candidate connect with you on social media so you can see what they post.