for the pre amp and phono amp, you will notice a sharp drop in gain, or perhaps a drop in audio quality. it would be significant. A turntable, the motor will slow down under load.
If I were to test them, I would put a VOM meter on the output and measure DC voltage, just to confirm. I'd also measure AC voltage. All under load. It's the simplest test i can think of.
There are others were you can measure resistance, and watch it reduce as it charges. But the time it takes to do that will vary depending on the VOM meter.