Hopefully you know, how a 10:1 passive probe can show the waveform right regardless the input capacitance of the oscilloscope and the probe wire. Make your own same doing compensation circuit. Connect the signal source to the input of the scope through a resistor and your sensor connected in parallel. Find a resistor which placed in parallel with your sensor shows square wave perfectly.
Let that resistor be =Rx. The unknown capacitance of the sensor is Cx. Solve the following balance-equation:
Rx * Cx = R4 * C2 where R4 and C2 are the input resistance and capacitance of the scope.
Be sure there's no long wires making unknown extra capacitance in parallel with Cx or C2. You can add an extra resistor in parallel with the scope input to make a smaller Rx be enough. Otherwise you must have Rx=several megaohms.