But how do you know you're right? Especially with celebrities, you can't tell their true personalities because they're always on in front of a camera.
No one can be a good enough actor. The true colors of any person can be seen easily if you have good eyes for people.
Without really knowing someone, how can you type them?
People have some characteristics that stand out in powerful contrast with their other characteristics. Every cognitive type has some recognisable characteristics. If you know them and you are used to them, you can easily observe and recognise them even in people who you have little knowledge about their "real" life.
As for knowing someone, you don't expect that psychologists like Jung spent some time with people, 'getting to know them a bit", about their "real" lives, in order to recognise and type them.
That would be very: 1. Impolite and improper and 2. Not very efficient.
Also, I am a INFJ, one of the few "real" INFJs I know are on this forum. In Socionics, INFJs are described as having the capacity or ability to recognise people's main characteristics and even idiosyncrasies.
That means when I see a ESTJ for example, sometimes only after two seconds after I see the person, I already know in my front lies a ESTJ person. They have a style that is unmistakable, they way they talk, the way they walk, the way they move, and finaly, the way they act. And so is with every other MBTI type.
I hope that answers to your questions.
I don't have the pretention that everyone I type is the correct type, but for most people I type, I can say with 100% certainty they are the type I am recognising in them. I can bet for that with anybody.
For example, if you and I we are to walk on a street and we would just talk with people, and you would say to me "What is this person's type?" and I would answer "This person is a ESFP!" and you will check my answer with a MBTI professional or a socionist (whcih implies the respective person will make some tests) you will see I am right.