Another problem is that the tenants may have lived in the place for only the previous few months and you are viewing it in January or February. They really don't know yet how much a winter's worth of oil costs...still, gather what information you can.

A final caution: Difficulties with the landlord could mean either the landlord is "the problem" or the current tenants are "the problem". Look around the place...does it look like it has been well-maintained over the years but has a few months worth of grime on it (landlord cares) or does it look like it has not had repairs, reflooring, repainting, new taps, etc. (landlord doesn't care)?

Sample questions to ask tenants:

Is the landlord prompt about doing repairs? Are they done well?

What do you wish you'd known about the place before you moved in?

Is there enough hot water for everyone to shower in the morning?

What's your rent per month? What do you figure your utilities (gas/electric/water) cost each month? Note: each billing period is two months.

How is the place heated? (Oil, gas, electric? Forced air or hot water radiators?)

If oil, how much have you spent so far this season? (Ask the tenants if they are willing to show you the bills.)

Do you blow fuses/trip breakers often?

May we get your phone number and call you if other questions come to mind? (This is a good idea, especially if the landlord is present; the tenants may not feel comfortable speaking candidly in the landlord's presence.)