You can ask just about anything. It would probably be best practice not to ask questions regarding protected or sensitive areas such as religion, race, family status, health issues, etc. Think of it like an interview of the candidate, the questions you would not ask the candidate (consult with your HR department if you are not sure which areas are off limits) you probably should not ask the previous employer. But the onus is really on the previous employer not to reveal any protected or discriminatory information.
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