with your sig gen in square wave mode inject a low level square wave to the input. With the amp loaded on the output with your 8 ohm load, you should also see a square wave, not as pretty as the input however. Carefully adjust it so that you get about a 2V P-P output signal
Using your scope measure the rising and falling edge time and voltage from the 10% point to the 90% point. For example if you measure a 1.6V rise in 0.8 microseconds then you will have a 1.6/0.8 = 2.0V per microsecond slew rate. These figures are just for example purposes. Clear?