You'd be much better off using attachInterrupt to get an interrupt whenever the change pump input has a rising-edge transition. On each interrupt, get the current time, using millis(), and calculate the time since the last interrupt, which will tell you the frequency. Compare that measured frequency to to some reasonable range. Nominal charge pump frequency is 12.5 kHz., so if you see transitions at a rate from 10-15 kHz, you can assume the charge pump is running.
There is also an Arduino library called FreqMeasure, which will directly measure the frequency of a digital input, and give you a frequency value, in Hz. The sample sketch for that library will give you almost everything you need, except for testing the measured frequency is in a valid range.
Regards,
Ray L.