My dog has hurt his leg how can you tell if its broken

The only way to be 100% sure is to get a vet to take x-rays. The reason is you may not be able to see a protruding bone or feel a broken bone, but that doesn't mean the dog doesn't have a hairline fracture. So there is really no way for you to be 100% sure. Do you mind me asking about what happened to the dog?