> What's wrong with the old trick of cranking the gain all the way up, > plugging no microphone at all in, and getting thermal noise?
Some modern soundcards have something like noise gate. From my PCI128 card I got about one bit of noise, and even that not consistently. Too high input impedance. Too easy to affect with external signals. Likely to pick up 50/60 Hz hum. Otherwise, it's good idea, but its application is likely to depend on the actual sound hardware used.
