How capacitive is the human body? Testing a capacitive screen, it's clear that it can tell the difference between a celery touching it (human holding it with an isolating material) and a human touching it. Touching it holding a celery, but no isolating material works too.
So, what is the range of 'normal' capacitive potential of a human body?
(I am not interested in capacitive screens, but rather on electrical properties of human bodies, however, the capacitive screen got me interesting on it). 
How could we measure it? 
 A: There is no such thing as inherent capacitance of a object.  Capacitance is a property between two conductors.  Your question therefore makes no sense.  Capacitance of a typical human to where?
In the case of touch screens, the sensors are measuring capacitance back to their own ground.  How well a human touching a sensor couples back to that ground has a lot to do with the surrounding physical layout.  Capacitive touch sensors typically work on sensing a change of a few pF, sometimes even less than 1 pF, over a few 10s of ms.
Added:
In reading this over I think I left the wrong impression on how capacitive touch sensors really work.  Actually, they only measure the change in capacitance as a conductive object such as your finger is put in close proximity to the sensor.  Such a sensor has two plates, sometimes interleaved, so that putting your finger close increases the capacitance between the plates.
This doesn't change the fact that the human body will have some capacitance to the local ground, which is more what this question is about.  Again though, there is no fixed inherent capacitance of a human body by itself.
A: If we assume a perfectly spherical and conductive human body, we need approximately 30 pC to raise the voltage by 1 V, that would be 30 pF. 
Only, as Olin pointed out, this quantity is not what is called the capacitance of a conductor, which, for historical or practical reasons, is defined for a pair of conductors. 
