A numerical method is presented to treat atoms in strong time-dependent electric fields. The method is applied to calculate ionization probabilities of H Rydberg atoms in strong monochromatic and bichromatic microwave fields. Recent experimental data on the microwave ionization of H Rydberg atoms are well reproduced. Ionization probabilities for microwave field parameters not yet covered experimentally are predicted. We propose to use the setup of existing two-frequency waveguide experiments to reveal the structure of the quasi-energy spectrum of microwave-driven Rydberg atoms. In analogy to dressed-state spectroscopy of atoms in strong laser fields, the quasi-energies of a periodically driven quantum system are mapped out by sweeping the frequency of a second (weak) additional rf probe field.