There are ways to do this.
In hardware:
What you do is split the voltage into two paths, one goes straight to a comparator circuit, the other goes to other input of the same comparator but via a low pass filter. Over time, the two voltages will be equal and the comparator circuit detects this. Because the LPF voltage is slow to follow the input, any instantaneous difference will be detected by the comparator. Eventually the LPF voltage will again be the same as the input and the comparator will cease to detect the change.
In software:
Use an ADC to measure the voltage and store the result, keep repeating this and comparing the new and old readings. You should allow a small difference because of ADC accuracy but basically, any significant difference in measurments indicates the voltage has shifted.
Brian.