Idris

Pour it away if it smells sharply of vinegar or has gone cloudy when it was not a cloudy style to begin with.

Worth adding that most everyday sake is not meant to be kept. Aged styles exist, but a standard bottle does not improve on a shelf for a year, and people treat it like wine and are then disappointed.

What good sake should taste like, for comparison: fruity and floral, or rich and ricey, or dry and crisp, or savoury. Some styles are noticeably acidic, but balanced and fresh rather than sharp and souring.