Answer: A person is said to be just when he performs all those acts which are obligatory upon him, and refrains from all those things which are forbidden to him. And the sign of being just is that one is apparently of a good character, so that if inquiries are made about him from the people of his locality, or from his neighbours, or from those persons with whom he lives, they would confirm his good conduct.

Answer: You must make sure that he is just, or else, you cannot follow him, also ensure the has he other conditions of leading the prayer for example reciting the prayer correctly.