First so you know , there are many rental units that were used as such before zoning changes came along in many areas to regulate them. You may find a unit with only one door to the exterior and it still is a legal unit because it was in use as one b4 rule changes. Sometimes in multi units like 2 unit structures one can find that the LL is including all utils , this alone does not mean a unit is illegal , it means that the LL never had the funds to pay for utility splitting and is fine with it as is by including the utils or a approved by state method of splitting them up. ( I have even seen a older 8 unit 2 blocks from my first home , it had all utils .elect,gas heat, hot water, cook inc) rather than its long time owner splitting it up) Now if your really curious the thing to do is call your city or county zoning desk ( only if you are prepared to be forced to move out) and give them your address and ask how many housing units the address is zoned for, and also check your county property tax desk, If the structure is zoned as anything above a single family home and is being taxed as something other than a single family home they can tell you, it is public info. Once you have done that do not be surprised if someone from inspections comes out , and wants to see it if it is zoned /taxed as a single fam and used other wise. They will order the LL to get you out.