Answer:

Yes, you may ask if a candidate is authorized to work in the United States and let them know that proof of eligibility will be required if they were to be hired. However, you must be consistent and ask this question of all candidates.

Additionally, be sure that you are only asking about authorization or eligibility not the candidate’s citizenship status or national origin. Citizenship status and national origin are protected classes which may lead to the appearance of discrimination should a candidate not be hired.