The typical Hello Kitty wears a red ribbon on her left ear.  Mimmy, her twin sister, wears a yellow ribbon on her right ear.  If you see an item with "Hello Kitty" printed on it and the "Kitty" wears a bow (or other accessories) on her right ear ONLY, then it's a fake.  I must stress this point with "ONLY" because there are times when both Kitty's ears have bows or other accessories like flowers on them.  For some series like the bridal Kitty items, there's nothing on her ears, only flowers or other accessories like a tiara, etc. in the middle.  So the simple rule is:

To be a genuine Sanrio Hello Kitty item, if there're the words "Hello Kitty" printed on the item itself and if there's accessory (be it a bow, a flower, a bat, a candy, ......) on one ear only, that accessory must be on her left ear.