I am currently analyzing sensor signals in MATLAB.
As you see FFT is done for the raw and filtered signal. This is just an exemplary filter, which I need to adjust.
However, I am having trouble deciding which signal frequencies are relevant and which should be filtered out. I do know that the highest peaks in frequency spectrum depict the most important, however they are not always a clearly visible as on the screenshot which I sent you.
Do you know any methods of determining the most relevant frequencies or maybe you could recommend some articles/literature which would explain it ?
We can't tell you what the important frequencies are, that's for you to decide. Why are you doing an FFT? There must be something you're trying to evaluate. Whatever is driving you to run an FFT should give you hints about what frequencies to look at.
You could look for fundamental frequencies, but that's just a guess. The harmonics and their relative magnitudes are just as important - they determine the timbre of the sound. A saxophone, a clarinet, a trumpet, and a flute may all play the same fundamental frequency/note, but they sound different because their harmonics generate different timbres.