How could you measure ripple frequency to determine if a diode were open in a bridge rectifier circuit?

Measuring ripple frequency would determine if a diode were open in a bridge rectifier circuit because the ripple frequency is normally twice the input frequency in a functioning full wave bridge rectifier. If one diode were open, the ripple frequency would only be the input frequency.

Note: This is true for single phase or bi-phase operation. Three phase operation is more complex, but still doable - You would expect three times input frequency in normal state, and two times (asymmetric) with one open diode.