How do we actually measure electret?

How do we predefine and measure electret, what unit practically is it supposed to be?

Welcome to ETO!
Your questions are unclear.
By 'predefine' do you mean select? What is the intended use of the electret?
The only thing you would normally want to 'measure' is its geometry/size.

Maybe you need to measure the sensitivity and frequency response of an electret microphone? Those specs and more are in its datasheet.
Here is the frequency response of a fairly inexpensive but high quality Name Brand electret mic. It is almost perfect: