While many presentations of limping behavior stem from temporary causes such as bumps, bruises, sprains, etc., any limp that persists beyond a week should be seen by your veterinarian. This is especially critical if you have a large or giant breed dog, as they are MUCH more likely to develop osteosarcomas than smaller dogs. Ideally, examination should occur within the first 24-48 hours of onset, especially if the animal is non-weight-bearing on that limb. Once in the company of your vet, ask them for an x-ray (even if they don't offer one). This is the only reliable early method for spotting this type of cancer as it does not show-up on routine bloodwork and manifests from the inside of the bone. An unfortunate fact is that many cases are discovered after a pathologic fracture occurs...and at that point it's usually too late for treatment as the disease has metastasized into the lungs or other organs. The price of a few x-rays should mean nothing to you when compared to these facts: up to 85% of all bone cancers in dogs are osteosarcomas, these truly are the most painful of all tumors and survival time when not treated is 3-4 months. Just do it.