Perhaps the best way to buy sake is to have tasted enough to know exactly what you are looking for, and find that label. Advice and recommendations go a long way too. But we all need to foray into the unknown and try new things at times.
When choosing an unfamiliar sake, there are often several numbers listed on the label. These are supposed to be helpful, but can only confuse and obfuscate if their meaning is unknown. The nihonshu-do is one such number. Listed on many sake bottles, or maybe on a tag on the shelf, the nihonshu-do is a number that is supposed to provide an indication of how sweet or dry a sake might be. Although the era when sake was classified into either the sweet or dry camps -- and nothing else -- is all but over, this parameter is still often encountered.
Technically speaking, the nihonshu-do is the specific gravity of a sake, or the density of the sake relative to pure water. In the fermenting sake mash known as the moromi, koji mold breaks down starches in the steamed rice into various types of sugar molecules. Most of these (those that are glucose) will be converted by the yeast into alcohol and carbon dioxide. Others (non-glucose sugars) will remain as unfermented sugars.