Universal principles, such as the law of conservation of mass, are discovered through what process?
How else would you establish this? Of course, the experimenter must perform the right experiments. The principle was perhaps discovered relatively late in human history, by Antoine Lavoisier and Mikhail Lomonosov in the 18th century. You would think that given its universality, and the absolute, unqualified nature of the principle, that it would have been discovered earlier in human history.
And by the way, while conservation of mass is a given condition for all chemical reaction, does it hold for nuclear reactions?