  We introduce and demonstrate a new approach to measuring coherent electron
wave packets using high-harmonic spectroscopy. By preparing a molecule in a
coherent superposition of electronic states, we show that electronic coherence
opens previously unobserved high-harmonic-generation channels that connect
distinct but coherently related electronic states. Performing the measurements
in dynamically aligned nitric oxide (NO) molecules we observe the complex
temporal evolution of the electronic coherence under coupling to nuclear
motion. Choosing a weakly allowed transition to prepare the wave packet, we
demonstrate an unprecedented sensitivity that arises from optical interference
between coherent and incoherent pathways. This mechanism converts a 0.1 $%$
excitation fraction into a $\sim$20 $%$ signal modulation.
