How do you know if an INTJ becomes your close friend? INFJ here
INTJ are very strategic and enjoy time alone so if they are spending a fair amount of time with you (a day or two a week) and they discuss anything personal with you than you have earned their trust and they value your company enough to say you are getting close or are close.
We are very happy to be by ourselves so if i let someone in my personal space (my home) and they can just chill and i don’t need to entertain them or even talk much then they are rare.
I love when a person can just be at my place but not bother me or talk to much or expect anything from me. That’s the kind of person I’ll tell they can come over whenever.