Umbilical hernias are not uncommon in puppies. In the vast majority of cases they close on their own by the time the pup is a few months old. Often what happens though is before the abdominal wall closes, a small globule of subcutaneous fat falls outside the abdominal wall and is trapped under the skin when the abdominal wall closes. This is what causes the little bump under the skin, as shown in the picture.
How to tell if the hernia is closed or not is simple. Palpate the area and push on the blob. If you can feel a hole in the abdominal wall under the skin, or the little blob of fat can be pushed back inside, it's not closed. If no hole can be felt and the little blob of fat can't be pushed back in, the hernia is closed and the dog just has an "outie".