In olden times, sakes were classified only as Dry (Karakuchi 辛口) or Sweet (Amakuchi 甘口) and the sake brewers have a guide called the Sake Metre Value (SMV) or Nihonshu-do 日本酒度 to measure the dryness or sweetness level. The SMV measures the percentage of alcohol to sugar in the sake. Therefore a higher number means more alcohol which results in a drier tasting sake. A lower or negative number will indicate a sweeter sake. Hence, from the SMV, you can know the taste of the sake before opening the bottle.
Check out the SMV 4 'Black Dragon' Kokuryu Jungin Junmai at our shop! It is said to be the sake favoured by the Japanese emperor.