if they look at an apartment in a house, find out if the homeowner has a valid permit (if applicable in your area). without a permit, they may need to move if the town finds out it is there.
there is also a very real possibility that fire codes were not adhered to with "illegal" apartments. even if a permit is not required, check that there are ways to exit rooms in case of a fire (this comes into play especially with basement apts), and fire detectors are installed.
as pps said, ask about parking (if they have 2 vehicles, make sure each will have a spot), and utilities.
visit at various times of the day (friends learned this the hard way, when they were awakened early M-F by the kids at the busstop in front of her house, and often blocked in her driveway by the buses).
if it is an apartment house, try to speak to the existing tenants. find out if they are happy there, if there are any noise problems, how receptive the landlord/maintenance is when there is a problem .(i.e., are the walkways cleared quickly when it snows, are things fixed in a timely manner, etc). of course, they can ask a tenant of a house-based apt, but it might be a bit more difficult.
definattely find out how much security is required, and how long the lease is good for (especially for coouples living together, find out the terms if they need to break the lease in case of break-up, etc).
as a previous landlord, remind them that they are also being interviewed. we had quite a few ppl who were interested in the apt in our home, and went with the ones that we felt were most mature, quiet, and responsible - we wound up with wonderful tenants!
good luck
