How can I assess the similarity between the frequency spectra of two signals?

I'm in the process of comparing the frequency spectra of two EEG signals to evaluate their similarity. My main goal is to determine whether both signals have undergone approximately the same filtering process and I'm interested in obtaining a numerical measure of their likeness. I am sure that are various approaches to do this but am unsure about the most effective strategy.

Visual representations of the graphs are provided for your reference. Any insights or recommendations you could provide would be appreciated. Thanks in advance.

[Mathieu NOE]

hello

as we don't have your code , I can't tell for sure , but maybe you did not normalize the output of your fft - because I see very large difference in the magnitude, the right plot shows roughly a 50 times larger fft output vs the left plot (contaminated by a 60 Hz mains rumble BTW)

so looking simply at those two plots I wonder what we could say about the two signals, probably nothing

otherwise you may be interested in this lecture :

Measure Signal Similarities - MATLAB & Simulink Example - MathWorks France