Everything Marc said except I'd run a slow log frequency sweep and some pink noise instead of just measuring at 1 kHz.
Set up one mic on a stand, then switch mics. Don't change anything - just record one channel. Set the playback volume and gain once only.
You can generate the test tones in cool edit pro and look at frequency analysis there as well.
I've also just started playing with the free RightMark Audio Analyzer - might be worth checking out if you have a audio interface for your pc.
http://audio.rightmark.org