Final answer:
The most accurate way of determining the quantity of product in an open bottle of spirits is to use a graduated cylinder or a burette to measure the volume of the liquid.
Step-by-step explanation:
The most accurate way of determining the quantity of product in an open bottle of spirits is to use a graduated cylinder or a burette to measure the volume of the liquid. These instruments are designed to provide precise measurements of liquid volumes. By carefully pouring the contents of the bottle into the cylinder or burette, you can determine the exact volume of the product.