Actually you do. The only time I see a need to set the gain using the radio's maximum output voltage is when you will be using a DSP, X-over, or Graphic EQ. You see what the radio can put out at 75% (100% for quality radios) volume, then set input gain on the device. After that, you keep radio volume at the 75% and set the amplifier gain using the speaker's resistance and the power you want to send to it.
So if you have a 1kW amp with a 500w RMS sub, you will set output for 500w RMS.
I do not know how to use an oscilloscope for setting gain. I only know you can see noise or clipping when the hardware is pushed too far.