I'd measure the amplifier voltage output with no load, and then connect a low value resistor across it to see how much that dropped the voltage. Monitor the voltage across the amplifier's output terminals.You can either measure current directly or you can measure the voltage drop across the resistor and calculate current. Then, knowing how much reduction in voltage on the output will tell you what the output impedance is. Divide this amount of voltage drop by the current flowing to find output impedance.