I guess you could use whatever time-domain rms or peak amplitude measurement you would use normally and compare both signals. then just divide the higher-than-nyquist volume by the total volume to see what ratio is going to be aliased.
for example if the volume of the would-be aliased part is 1 and the volume of the total signal is 2 then you know that 1/2 of the signal will be aliased
one thing to consider when using a highpass vs. using an fft is that the sine-wave components will have their phases changed by the filter, so the amplitude might not be reflective of the actual amplitude of the aliased signal (so maybe rms is better since it uses averages, so the phases won't matter as much I think).
if you want to measure using amplitude I guess it might be best to sum the FFT magnitudes of each bin and/or use some kind of spectral/FIR brickwall to separate the 2.. but rms might be a better measurement anyways
honestly idk that much about it & am just guessing maybe someone better at dsp will chime in..