If, G-d forbid, you or a loved one has been bitten by a dog, and suffered personal injuries as a result, that alone isn't enough for you to win a dog bite case against either the dog's owner or the landlord of the building where the dog was kept. In either case, you will be required to prove (among other things) that the dog had exhibited "vicious propensities" before the attack.
So, "How do you prove that?", you ask.
Fortunately, New York's courts have given fairly clear guidance on the subject. In fact, in a February, 2010 opinion, one of New York's appellate courts summarized the required proof as follows:
"An owner's liability for a dog or attack is determined solely by application of the rule of strict liability for harm caused by a domestic animal whose owner knows or should have known of the animal's 'vicious propensities' ( see Petrone v. Fernandez, 12 N.Y.3d 546, 550, 883 N.Y.S.2d 164, 910 N.E.2d 993; Collier v. Zambito, 1 N.Y.3d 444, 446-447, 775 N.Y.S.2d 205, 807 N.E.2d 254; Hodgson-Romain v. Hunter, 72 A.D.3d 741, 899 N.Y.S.2d 300). "Evidence tending to prove that a dog has 'vicious propensities' includes a prior attack, the dog's tendency to growl, snap, or bare its teeth, the manner in which the dog was restrained, [the fact that the dog was kept as a guard dog], and a proclivity to act in a way that puts others at risk of harm" ( Hodgson-Romain v. Hunter, 72 A.D.3d 741, 899 N.Y.S.2d 300; see Bard v. Jahnke, 6 N.Y.3d 592, 597, 815 N.Y.S.2d 16, 848 N.E.2d 463; Feit v. Wehrli, 67 A.D.3d 729, 888 N.Y.S.2d 214; Galgano v. Town of N. Hempstead, 41 A.D.3d 536, 840 N.Y.S.2d 794)."
As you can surely imagine (and this is borne out by the sheer number of dog bite cases that New York's courts have summarily dismissed), this is not a simple thing to prove.