Is there a method to measure the output impedance of a condenser microphone?
Of course. First you must have a repeatable amount of sound. Measure the output amplitude without any load (a high-impedance load, like a o-scope probe, is OK). Next load the output with a resistor equal to your best guess of the output impedance, let's say 1k. Measure the output amplitude again. If it is half the original value your guess was correct. Otherwise solve the equation
B * 1k = A * ( 1k + X )
A = open amplitude B = loaded amplitude 1k = the load resistor you used X = output impedance
Note that, apart from the requirement for a repeatable sound level, this is the same procedure you would use to find the impedance of any voltage source.