A good way to tell if your dogs leg is broken and not just sprained is to watch the way the dog walks.
A dog who has a broken leg will usually keep their foot off the ground and not want to put any weight on the leg so you'll usually notice the dog has it's foot in the air when it's walking on the other legs.
The dog will usually be in pain when they have a broken leg so they will not want to put any weight or pressure on that broken leg.
If you notice your dog has it's leg in the air while it's walking or is maybe refusing to walk then the dog could have a broken leg.
If the dog has just sprained it's leg then it will usually walk with a limp and they will usually have their foot or paw on the floor or ground but they will usually walk with a limp.
If you think your dog has broken it's leg then you need to see a vet to have the dogs broken leg healed.
If your dog has suddenly stopped using it's leg or is limping and has nothing in it's paw that is hurting it then it's likely the dog has sprained it's leg.