What observation of seismic-wave travel led to the discovery of Moho?
The Moho was identified from seismic wave measurements. It's named for the seismologist who discovered it in 1909, Andrija Mohorovičić.
Croatian seismologist Andrija Mohorovičić discovered that seismic waves could take two paths between points close to the Earth's surface. One is the direct path through the crust. The other is a refracted path that detours into the outermost part of the mantle; this second path is longer but the waves travel faster through the mantle rock.