Simulating (transient) intermodulation distortion
The test you are talking about involves the harmonics generated through IMD, which appear as differences and multiples of them around the carriers and harmonics of the carriers. I would refer you to hypex.nl, where you can browse for some nice documents, to read about them.

As a schematic, here's a quick mockup in LTspice:



You can see the input is made of a voltage source, summed up with a current source (which is making use of V1's internal Rser=1), to generate a 1Vpk test signal comprised of 19kHz and a 20kHz sines. The runtime is set to be a multiple of their difference, 20kHz - 19kHz = 1kHz => 1ms period. Here, 16ms. In addition, the almost mandatory plotwinsize and numdgt settings and a timestep of 1/2^10/f, for a binary division, even if that doesn't really matters sine LTspice a) doesn't use a true fixed timestep and b) it uses its own interpolation system, internally. But, it's good to be on the safe side.

To visualize the effects, one performs an FFT on the waveform, with these settings (as chosen here):



Note that binomial smoothing is set to 1 (disabled) and there is no wondowing applied (sharpest response). The result:



You can clearly see the IMD's effects around the two carriers. For more exotic (and bad) circuits, you can also see harmonics around the beginning of the spectrum (lower frequencies), I think there are some examples at the site I was talking about.

Maybe you could use other methods, as well, but this should be the most straight-forward and give you the clearest picture, IMHO.