  We probe the transition between superfluid and Bose glass phases using
quantum quenches of disorder in an ultracold atomic lattice gas that realizes
the disordered Bose-Hubbard model. Measurements of excitations generated by the
quench exhibit threshold behavior in the disorder strength indicative of a
phase transition. Ab-initio quantum Monte Carlo simulations confirm that the
appearance of excitations coincides with the equilibrium superfluid--Bose-glass
phase boundary at different lattice potential depths. By varying the quench
time, we demonstrate the disappearance of an adiabatic timescale compared with
microscopic parameters in the BG regime.
