RTA software can be used to replace an oscilloscope and provide additional functionality for looking at electrical signals. This video explains the process of measuring amplified audio signals. Note that you need to know the input voltage range of your PC or device running the software. This video talks about reducing the signal voltage for use by the device.

I used a David Navone Engineering LOC in the test because it represents an attainable solution for the viewers. I spend time explaining what i'm doing and why so the test is repeatable, I apologize for the length. Clipping harmonics are at 9:45.

UPDATE - as a follow up to some comments on the appearance of the clipped output, I did test both the Alpine IVA-W200 shown and an Alpine DVA-9861 from the RCA outputs directly into the RTA program and USB interface as well as the BK Precision scope in the background. Results were similar even without the LOC or crossover. I suspect the waveform shape is mostly due to slew rate in the audio signal path.