The simplest method would be to place a 100 K resistor from 5V to the input pin you want to measure and checking to see if it goes low when finger contact is made, however allowing direct contact to an input pin on an Arduino isn't a great idea, hard to protect the device as it is hard to protect from accidental static electricity discharges. an interface of some kind is in order but I think you need to do some reading first and decide what you are going to do. I can suggest a couple of inexpensive and bullet proof (as far as the Arduino goes) methods of interfacing a pin to the outside world but I think you need some more thought about what you want to do. Under any circumstanced you need to either pull up the pin and let the finger contact bring it down or do the reverse but you will get no real results with an open pin as it doesn't have a predefined state to change from. If the Pin is neither high nor low then what information is present when it changes state??? and how do you tell??? IMO
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