Mdd
“How does one know that a person is a tzadik in the Tanya’s definition? In Shamaim they know, but people?”
The difference between a beinoni and tzadik is that a beinoni has a yetzer hara (and doesn’t give in ever) while a tzadik doesn’t because he either put it to sleep (tzadik vra lo) or transformed it to another yetzer Tov (tzadik vtov lo).
You’re right there’s no yetzer hara xray- but you can tell usually from the way a person expresses themselves, whether they struggle with evil or not. If you’ve met/ experienced a tzadik- you would know- they’re on a different plane