There are many causes for joint issues in dogs, such as genetics, injuries and disease. An overweight dog is a also highly predisposed to joint issues. Preventing joint issues can be done by keeping your dog at a healthy weight and ensuring that your dog's parents (especially if you are rescuing) are OFA-certified, meaning they're genes have been verified as healthy.
You'll know that your dog's joint issues are more serious when they're moving slower when getting up or lying down, going up or downstairs, jumping or are not enthusiastic to go on walks. According to Dr. Laura Robinson, DVM, most orthopedic issues are not necessarily dangerous, but can be extremely painful. Dogs and cats have a very high pain tolerance, which unfortunately means that humans will not notice their pain until the disease or issue is advance.
At the first sign of lameness, limping, or slowing down, keep your dog comfortable and get them to the vet as soon as possible.