Create a paper trail of dated government documents and medical bills to indicate duration of pet ownership. Save the adoption certificate, license, bill of sale, or American Kennel Club papers you receive. Bring the dog to the veterinarian immediately. Save all bills and records of inoculations, rabies tags, spaying or neutering certificates. Take pictures of the pet in the apartment and in the building, and date them when you get them from developing.
Every picture is worth a thousand words, so be creative.
A photo of you, the dog, and the doorman next to the building Christmas display would show that the building's agents were aware of the dog at a certain point in time, for example. At least one case has been proven by the tenant's keeping careful photographic records of her dog's growth, from tiny puppy to full grown dog, in the apartment and around the neighborhood. Save any letters or complaints from the Board or management pertaining to the pet. Keep a log in a notebook, used exclusively for this purpose, in which you record any date that workers were in your apartment and saw the dog, or if the super or doorman saw the dog. Make sure that all entries are made on or near the time of the occurrence.