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It's not always shorting (voltage) or opening (current) the input sources. If the input source impedance is finite, like eg 50 ohms, you should connect this impedance to the input node.
Then again you can define the output impedance with the calculation stated before.
Drive the circuit with say, a 1K sine wave, and measure the open circuit output. Then apply a known load, say a 1K resistor, and again measure the output amplitude. Knowing the no load and loaded amplitudes, you can now calculate the output impedance at that frequency.
Zout = Rload(Vopen - Vload) / Vload
Impedance is frequency depedent. To add more in addition maybe later you would consider temperature is device under test being measured, other contributing noises etc.
btbass has good rule of thumb without complexty at all for measuring the impedance. This may not be very accurate but it is good way to start.
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