Ask in advance if there are tenants and if there are leases, what the rents are etc.. You may not be able to see the lease until you make an offer but at least you will know. If the property is vacant, check Craigslist etc to find out what you can reasonable expect the rent to be, don't just rely on your realtor but ask around. I don;t know what they have in New York for landlords but in Boston you can get a book about being a landlord, lease agreements and other forms. You want some of those NOW so you are familiar with them and the landlord/tenant law.
Something to find out about on your viewing is how the utilities are set up. That will effect what and how the tenants are charged. For example, is there one one oil tank and furnace that both units share? Is there only one electric meter? In MA there should be three meters for a two family, one for each unit and one for the landlord for common areas. Can these issues be fixed or changed, yes but its something to know going in.