You usually need a veterinarian to tell you what's wrong with your dog.
You may notice signs that something is wrong. Your dog is limping, or not eating, or drinking more water than normal. You may have an idea of what it could be, but you will probably need a veterinarian's exam, and possibly some laboratory tests or radiographs (x-rays), to make a diagnosis.
But how do you tell what's right with your dog?
In positive reinforcement training, a cue is something that indicates to the learner (your dog) that you would like her to do a certain behavior.
Most cues in dog training are verbal or visual. But cues can be olfactory (a dog training in scent work sniffs the odor and sits) or auditory (a click in clicker training), or environmental (you take your dog out of the car at the trailhead, and she knows she is going for a walk). A touch can also be a cue.
The dog learns that when she hears, sees, smells, or feels the cue, and performs the correct behavior, she will earn a reward.