ADA's basic message is that you can't ask pre-employment questions that could reveal the applicant's disability. But the law sets different rules for different stages of the hiring process, including:
Pre-offer. ADA says you may not ask any disability-related question or require any medical exams, even if they relate to the job.
What counts as a disability-related question? The government says it's one that "is likely to elicit information about a disability." To be safe, ask questions only about the person's ability to perform the job's essential functions.
Conditional job offer. The rules become less cumbersome once you conditionally offer the applicant a job (but before he or she starts work). At this point, you can legally ask disability-related questions and conduct medical exams. But to preserve the right to ask these questions, you must subject all applicants in this same job category, regardless of disability, to the same questions or exams.
If you decide to withdraw the job offer at this point, you must be able to show that the reasons disqualifying the applicant from consideration are "job-related and based on business necessity." That essentially means that the person will be unable to perform the job's essential functions because of a medical condition, or the condition would present a threat to others.
After employment begins. At the third stage (after employment begins), you can ask employees disability-related questions and require medical exams if those queries are job-related and consistent with business necessity. That includes instances when you think the employee would be unable to do the job's essential functions due to the condition, or if the employee has requested an accommodation for the disability.
Note: Find a Q&A on the government's disability-related interview rules at www.eeoc.gov/policy/docs/qanda-inquiries.html.