You can try to play a "white noise" signal, apply eq, record it back to PC and analyse it with FFT.
I think you can use audacity both for generating a white noise signal and for recording back the effect of the equalizer.
A white noise signal is a kind of random signal that has the property that all frequencies in the signal have the same energy (on average). So if you play back a white noise signal, boost a specific band, record that back and analyse it, you should be able to see a bump in the spectrum around the centre frequency of that band. Repeat for all equalizer bands.