No, there is no logical method I can think of for a program to determine if a returned ADC count of say 200 was the result of a valid input voltage of that value sent by a sensor or of the results of a disconnected sensor.
Well in this project:-
I have to decide if the spoon is touching the conducting pad or not.
The spoon is connected to the analogue input pin and each end of the conducting surface ( let's just consider one of the two dimensions here ) is connected to an output pin.
The method I use is:-
Discharge the sample and hold capacitor in the A/D by switching the input multiplexer to ground.
Then
The two outputs to one - if I measure anything below a large threshold it is floating
Then
The two outputs to zero - if I measure anything above a small threshold it is floating
If any of those two tests say it is floating then I assume it is.
If not floating then one output to one and the other to zero and take the reading, which is proportional to the spoon distance on one axis.
I then make the two outputs into inputs and repeat the measurement only for the other axis.
Maybe you could adapt this technique to your situation.
Having said that capacitance measurements on an arduino is flaky at the best of times.