Technically, the only question it is ILLEGAL to ask prior to offer is anything pertaining to the ADA, with the exception of: these are the job duties, will you have any problems with performing them? However, if you ask anything that would relate to age, marital status, children/no children, religion, national origin, etc., and the applicant does not get the job and sues for discrimination, just try and prove that you did not take the answers to the questions into consideration. Anything of that nature should only be asked post offer. If you must ask it at all.