I am working on decimation of signal, and I want to know which is the best way to understand if the downsampling is well done or not. At the moment I am comparing the FFT of the source signal and the downsampled signal and I observed a downward shift of it (I think it is due to the lesser quantity of samples), I had also a look into the time behavior. Do you have other advices/suggestions?

**EDIT**

From the comments/answers:

* one method to understand if the downsampling process is well done is by comparing the FFT of the decimated signal and the original signal. The original signal must be attenuated by the decimation factor as suggested in the answer from @user2934229. Otherwise the signals can be normalized by the length of the signal (not suggested in my case because the signal is too long);