You just need to consider what it is you're trying to measure, and give it a little thought.
Then, with a signal generator, an AC-millivoltmeter (or a scope), and a resistor or two it's simple to measure.
To measure input impedance simply feed the signal generator through a series resistor to the input of the amp. Then measure the signal level at either end of it. It's then a simple matter to calculate the input impedance of the amp (two resistors in series - with the external resistor the first one). If you want to make it dead easy, use a variable resistor instead, and adjust it until the output side of the resistor is exactly half the input - then disconnect the pot and measure it with a meter.
You can do the same at the output, except the resistor goes from output to ground, and you measure the output voltage with and without the resistor - again if you use a pot, adjust it until the output is reduced by half (but don't do this on a low impedance output, such as a power amp).
Remember, it's a simple two resistor potential divider - for input you're trying to find the bottom value, for output you're trying to find the top one.