Working dogs perform a variety of essential services for their human partners, from assisting people with disabilities, to Search and Rescue, police, and military work. Recent interest in the nascent field of Animal–Computer Interaction has prompted research in computer-mediated technology for communication between working dogs and their handlers. Haptic (touch) interfaces in the form of vibrating motors are a promising approach for handler-to-dog communication. Haptic interfaces can provide a silent, long-range method of sending commands to a dog, when voice or hand signals are inappropriate or impossible. However, evaluating haptic interfaces for dogs, who cannot self-report sensations, creates interesting challenges. This study draws on human–computer interaction concepts, such as Just Noticeable Difference, to explore methods and issues in evaluating haptic interfaces for working dogs. We created a haptic system and developed an evaluation method, reporting results for ten dogs of widely varying breeds, sizes, and coat types.