Write an article based on this summary:

Look at the puppy's belly. Look below the bellybutton or umbilicus scar. Check for testicles. Feel the puppy's belly. Examine the puppy's rear end. Disregard the nipples. Consult your veterinarian.
You will probably see a "belly button," or umbilicus. This is usually located almost at the center of the belly, just below the rib cage. If the puppy was born within the last few days, the umbilical cord may still be attached. Once the umbilical cord shrivels and falls off -- which should happen within a few days -- there will be a little scar left behind on the belly. This scar is a slightly brighter color than the surrounding skin and feels a little bit thicker. If your puppy is male, there will be another small raised spot or "button" of flesh about an inch or so below the scar. This is the prepuce, or sheath, of the puppy's penis. The prepuce will have a small hole at the center. The prepuce may have little wispy hairs around it or even on it. Do not attempt to pull out or unsheathe a male puppy's penis until the puppy is at least 6 months old. Dogs have an os penis, or "penis bone." You may damage the penis or penis bone if you attempt to forcibly unsheathe a young male dog's penis. Male puppies will have testicles, although you may not be able to feel them before they are 8 weeks old. If you can locate the testicles, they will be high up between the puppy's hind legs. Depending on your puppy's size, its testicles will be about the size of a lima bean. By 8 weeks old, the testicles are usually enclosed in the sack-like scrotum. Unlike male puppies, female puppies will have smooth bellies (other than their umbilicus scar). Females do not have a prepuce. A puppy's anus is located directly under its tail. If your puppy is male, you will only be able to see its anus. If your puppy is female, you will see a raised, fleshy area just under the anus. This is the vulva. The vulva of a female puppy is small and leaf-shaped. It has a vertical slit. It is usually located almost directly between the puppy's hind legs. The vulva may have some wispy hairs on it. Just like humans and other mammals, both male and female dogs have nipples. They will not inform you of a puppy's sex. Puppies should have their first set of vaccinations at around six weeks old. If you can't figure out what sex your puppy is, your veterinarian can help you with this during a routine visit.