Contact a veterinarian promptly if your Newfoundland has sudden weakness, collapse, difficulty breathing, marked exercise intolerance, severe pain, or cannot comfortably use a limb. These signs can have many causes and need an in-person assessment.
For less urgent changes, such as a new limp, reluctance to exercise, or a gradual change in energy, arrange a veterinary appointment and note when the change began, what you have observed, and whether it is getting worse. Do not start medication or restrict activity beyond common-sense safety measures unless your veterinarian advises it.
Elbow dysplasia and dilated cardiomyopathy are listed as concerns that can occur more frequently in purebred dogs. That information is a reason to stay attentive, not a way to diagnose an individual Newfoundland.