How to determine the frequency of input signal from Sensor to ADC?
I think you need to look at the response time as given in the datasheet. 
If it takes the sensor 100 milliseconds to respond to changes in the pressure, then a cutoff of 10Hz would be reasonable. 

If you are using a low speed ADC, they often include oversampling and implement an antialiasing filter for the real sampling rate in software - a simple RC low pass for 10Hz would then do the job.