A student tests an electrochemical cell designed to recover copper from a waste solution. One half-cell contains a \(\text{Zn(s)}\) electrode in \(1.0 \, \text{M}\) \(\text{Zn}^{2+}\text{(aq)}\), and the other contains a \(\text{Cu(s)}\) electrode in \(1.0 \, \text{M}\) \(\text{Cu}^{2+}\text{(aq)}\). The half-cells are connected by a \(\text{KNO}_3\) salt bridge, and no external power source is used. The student proposes that the following half-reaction occurs spontaneously at the zinc electrode.

\[ \text{Zn(s)} \rightarrow \text{Zn}^{2+}\text{(aq)} + 2e^- \]

Which observation would provide the most direct experimental evidence that the proposed oxidation half-reaction occurs spontaneously at the zinc electrode?