> try driving the mic with a small voltage as you would with a loudspeaker.
Risky advice. Common dynamics will stand many dozen millwatts this way; one is even rated for speaker use. Ribbons tend to be an order of magnitude more fragile, especially to "thump". They have higher electromechanical efficiency, but much softer suspension.
> the voltage readings were very low but the mic works fine.
Output level of a ribbon may be 0.1mV at 74 dB SPL, 1mV at 94 dB SPL which can be uncomfortable.
> the voltages are too low to read on a DMM
DVMs are for mechanics.
A good 'scope will resolve ribbon levels, but many general purpose 'scopes won't show enough wiggle to read clearly.
You NEED a preamp.
Wire a 10K audio pot as a 2-terminal rheostat, 10K at max, 1K in the middel, zero at minimum. Add a paper scale, use the ohmeter to calibrate the knob.
Take a shell off a mike cable and tack the pot to the hot wires. Set the pot to 10K.
Plug this to your mixing console mike input. Verify that meters move, and that the zero-ohm setting kills the signal.
Put that mighty-loud Ricky Martin or Ramones CD in the boombox, turn it up to clipping, set the mike near.
Set the mixer so that racket hovers near reference level; zero VU or -15dBfs.
Turn the 10K pot until meter shows 6dB less. Read the pot scale.
This is approximately the broadband audio impedance of the mike.
For more accuracy, you should determine the input impedance of the mike amp and do math. Most inputs have Zin much higher than common dynamics, so it is hardly worth the effort. Some ribbons are cheated-up to high impedance. If the pot reading is 300 ohms or less, it's good enough. If the pot reading is higher than 300, you may want to correct. If the pot must be set to 1K for 6dB drop, and the mike input is 2K (a common value), then it is really a 667 ohm load and a 667 ohm mike. Since you never need to know mike impedance better than 20%, this is just hardly worth caring about.
Mike impedance is not constant with frequency. But it will tend to be very nearly flat. But narrow peaks or dips can upset sonic balance when used with lo-Z preamps. If that's what you are really asking, an electrical measurement is a lot easier to set up than an acoustic measurement.
Condensers need to be powered-up, obviously. Most condensers are padded-out enough that Zout is mostly resistive, with some transformer or cap error at extremes. But a few have near-zero Zout. These will not load-down, they will just distort, which is not what you want to know.