Good research is essential to maximize your chances of winning your case and protecting your rights. In Landlord Tenant litigation, you should always check the following three sources for information that can help you.
First is Homes and Community Renewal (formerly known as the DHCR, now called the "HCR") to get a copy of your apartment's rent history. This helps to determine if there have been overcharges. It also is crucial in establishing if there's a right to renew your lease, when the landlord claims there isn't. You may be entitled to rent regulated protection without even realizing it. If you notice large jumps in the rent rate, or gaps in the rent history, it would be to your advantage to see a Tenants' lawyer about possible violations.
Second is the Department of Housing Preservation and Development ("HPD"), for a list of violations on public areas of your building and on your apartment. While you are there, also check the "i-cards." These will give you historical information on buildings. They're an especially useful source of data for older properties that don't have filings at the Department of Buildings, like many of the brownstones in Brooklyn, for instance. HPD has a lot of information on its website.
In addition, under certain conditions, active Housing Maintenance code violations can mean no rent is due. A pattern of negligence is strong evidence to support your claims.
If your building has three or more separately occupied units, look for a valid Multiple Dwelling Registration, or "MDR." Without a proper MDR, nonpayment suits in multiple dwellings get dismissed, along with holdovers in Manhattan and the Bronx (holdover cases involve lease violations or renewal disputes).
Your third research source is the Department of Buildings ("DOB"). Here you'll want to get a Certificate of Occupancy, if it exists. If your building was constructed or structurally altered after 1938 and there's no C of O, once again the landlord's nonpayment case should be dismissed.
There may of course be other research to do on any Landlord Tenant case, but these three sources are important and sometimes overlooked.
Research doesn't always turn up information that is relevant to a case, but you won't know that until you do it. On the other hand, you may find exactly what you need to give you the winning advantage. Consequently, it's an essential effort that's well worth making. If you don't feel you can handle it on your own, speak to a Tenants' lawyer. Good attorneys thoroughly research a client's case in whatever ways are appropriate.