I have two electric signals from different sources and I think that are similar. I have the FFT of the signals and I have their spectrums of frequency in separate arrays. How should I proceed to find the percentual of similarity the two signals?
You could measure the Cross-Correlation between the two signals.
Here's an almost identical thread, with a filling answer:
Some general info on cross-correlation from wiki: https://en.wikipedia.org/wiki/Cross-correlation
Hope that helps.