How do you measure amplitude of a ripple?

Most true RMS voltmeters can measure the value of a ripple voltage on top of a DC supply, when you place it in AC mode. You can also place a small capacitor in series with a DC voltmeter and that would measure the ripple. The real way to do this, because ripple voltage is not sinusoidal, is to use an oscilloscope, particularly if you want the peak values.