We don't.
But for those who wish to ... There are thousands of tests out there, to "define" your "personality type".
How do we typically measure personality?
Walt is right as there is no measurement system for personalities as personalities are just what they are but we do know certain types of personalities make a person and act in a certain way under certain circumstances and that every one of us is unique and different in our own ways.
We can't. We are all unique in our own way.
I don't know how one would "measure" personality.
On the other hand that are a number of categories of personality types that can be useful.