Definitions of concepts

In several places you wrote about essential definitions versus definitions that are not essential to the matter.
Question: a) Is there an indication, method, or way to stand and distinguish between the types of definitions? For example, that the essential definition of a table is not a 'rectangular object' but an 'object used to place things on it'. Perhaps these definitions are incorrect, but for the sake of the example. Is there any method or skill to stand and know what the essential versus the external boundary is? b) Following on from this, how do you stand on the essential boundary? Is there a rule for this? For example, the term 'democracy', when will I know that I have understood the essential and not the external level? Is there some sign or is it purely intuition?