well,subduction is proved... First of all, seismology has permit to compare the velocity of seismic waves into the globe and we can report some anomalies : this is the seismic tomography (a cold material is seen beneath the volcanoes). Secondly, the sea level is slightly higher (about 1 meter) near the continents, (i don't know the world in English for that but in French well call it "bombement avant fosse"). It's totally prove subduction