Start with your country's official online tariff database or published tariff book, search by product description, and narrow down through the chapter and heading structure until you find the most specific matching subheading. Many customs authorities offer a binding advance ruling: you submit a full product description and receive an official, binding classification before you ever import, which removes the guesswork and the risk of a dispute later.
When a product is genuinely ambiguous, or the duty difference between two plausible codes is large, ask your customs broker or file for an advance ruling rather than guessing. The cost of getting this wrong, in back duties, penalties and delay, is far higher than the cost of asking first.