The two tone IMD is probably the simplest method, another method we used in early versions of Cadence was to sweep the input power and plot the fundamental vs the 3rd harmonic. Both methods however require an FFT, the first just a single FFT and the later many.
Remember doing both awhile back to get some idea of what to expect with actual hardware, one such circuit had some unique properties like gain expansion rather than compression at higher input levels.
Best,