The easiest way to measure ripple is to connect an oscilloscope, AC-coupled, to the supply output. Start with the attenuator set for a high volts/div setting before connecting the probe to the circuit and then lower the setting until you can observe a measureable waveform. As David mentioned, the ripple on a good, modern, electronically-regulated supply will be in the low millivolt range. The scope will be measuring peak-to-peak voltage. Ripple may be spec'ed as rms units or p-p units. Tektronix tended to use p-p units -- go figure!
Dean