The reactivity of metals like zinc, iron, copper, and lead can be determined through their reactions with acids or water. The activity series of metals provides a qualitative ranking of their reactivity. More reactive metals, such as zinc and iron, readily react with acids, producing hydrogen gas and metal salts. Copper, which is less reactive, may react with strong acids under specific conditions. Lead, being relatively unreactive, does not react with dilute acids. Observing the intensity of reactions or the absence thereof helps establish the relative reactivity of these metals, assisting in predicting their behavior in various chemical reactions.