Audiophiles spend a lot of time and effort worrying about audio specs like Total Harmonic Distortion (THD). Makes sense, because THD affects the quality of audio reproduction. However, THD can also affect interference from radio signals and even losses in power transfer systems. A simplified definition is the THD is the ratio of the sum of the power of all harmonic frequencies to the power of the fundamental frequency.
If a circuit produced a perfect sine wave, there would be no harmonics. There are many ways to measure THD in practice, but [Michael Jackson] has an interesting video showing how he easily visualizes THD using LTSpice. Assuming you already have the system in question in LTSpice (or you could use another simulation tool, if you prefer) it is fairly straightforward.
The key concept is to find what the expected output is. In [Michael’s] case, he had an amplifier with a simulated input, so he made the load impedance very high to find the actual voltage gain. Then he simulated a perfect differential amplifier and a perfect source that matched the expected output of the amplifier.
The rest is probably pretty obvious. You replace the load and look at the output of the differential amplifier. You can do an FFT on it to gain further insight into what’s happening. Of course, as good as Spice is, it isn’t reality. Your results in the real world may vary. But the technique is great for at least looking at (and presumably correcting) potential sources of high THD.
If you are into tube amps, THD is a critical subject. Modeling a tube amplifier is probably a lot easier than building it. Wiring up a plate supply and punching holes in a chassis only to find your THD is higher than you hoped would be a lot of wasted effort.