The voltage reading is the result of a combination of inductive and capacitive coupling of the wiring in series with the high input impedance of the meter. The gut feeling about how much coupling you have can come from the percentage of the source voltage that you read and a look at your meter specs. In other words, if you have a 10 Mohm input z and are reading about 40 volts out of a 120 volt supply, you are probably looking at about 20 Mohms of coupling impedance. The voltages aren't purely additive though because of the voltage phase shifts across reactive components. This just gives you a ballpark concept of how much coupling you have.
Now folks here will start suggesting that you use a wiggy or Simpson 260 or some tester. I still think you would learn more by inserting known, parallel resistances, with your meter leads. How hard would it be to throw a couple resistors into your meter pouch. Consider pinching your meter leads and noting your skin resistance.(NO MEGGERS PLEASE!) Throw a resistor of about that value into your pouch as a test value. Now you have a ballpark idea of how much voltage might be dropped across you if you were added to the mix. But we're talking learning here, not job productivity.
Joe
