You haven't said what's connected to the output of this sensor, but if it's a microcontroller and if you managed to get an analog sensor (of any kind) working, you could have the system monitor its health. For instance, when the pump starts up, you should measure a rising level in the tank. If that doesn't happen, something is wrong--the pump is jammed, the intake is out of the water, or the hose has come loose and instead of filling the tank, you're flooding the basement. Or if the tank fills properly but isn't drained in increments as the toilet flushes, there may be a leak.
The sewage holding tank might be more of a problem, given the nature of the substance to be sensed.
The sewage holding tank might be more of a problem, given the nature of the substance to be sensed.