Good Afternoon!
Here are a few to get you started. Get specific and ask follow ups. These are somewhat general...
Did they pay on time? How frequently were they late?
Did you refund the whole security deposit or were there repairs?
Were there any complaints from neighbors or police?
Would you rent to them again (why/not)?
Were they reasonable about repairs when needed (time needed, facilitating access)?
Did they abide by the rules of the agreement?
Why did they move?
Two other small pieces of advice that are gold, as far as I'm concerned:
After you've covered the basics ask, "Is there anything you can tell me off the record?" You will be surprised how often that will get you more information...often critical information.
Second, if at all possible, try to get on the phone with the landlord prior to the current landlord. If they are a bad tenant, the current landlord might say anything to get rid of them, but the landlord prior is under no such pressure.
Good Luck. Let us know how it goes!