Tap off the input via a resistor, feed to a cap with the other side of the cap going to ground and run the input and the cap/resistor combination to a differential amplifier.
When the input is at a stable voltage, the two inputs will be the same and the diff amp will be at zero output volts, if the input fluctuates then the diff amp will fluctuate accordingly less the DC value stored in the cap.
Size of resistor and cap depend on the frequency you're measuring.