The best way to tell is through comparing the Auxiliary functions Fe vs Te...
http://www.typexperience.com/video/57/INFJ-and-INTJ--Whats-the-difference
Te is a need to efficiently plan something.
Fe is a desire for human connection.
INFJs and INTJs both share Ni Dominance, and that can make us appear
very similar, even to each other. We also share Inferior Se, which gives us a similar approach to processing all around.
However, I've found that it is even easier to discern the difference if you consider the Secondary and Tertiary functions as a set.
INFJs have Fe and Ti.
Fe and Ti mix to create a support for Ni that is emotionally macro, and logically micro oriented. Fe focuses more on others than the self, emotionally. Ti sees the detailed workings more than the big picture.
Fe asks "How can I help?" Ti asks "How does this work?"
Fe is concerned with doing the right thing. Ti is concerned with what makes something tick.
INTJs have Te and Fi.
Te and Fi mix to create a support for Ni that logically macro, and emotionally micro oriented. Te sees the big picture more than the detailed workings. Fi focuses more on the self than others, emotionally.
Te asks "How does this relate?" Fi asks "How do I feel about this?"
Te is concerned with understanding how something can be applied. Fi is concerned with being a good person.
To tell INFJs and INTJs apart, the best way is to determine how each one uses their logic and their emotions, because we often come to the same conclusions and have similar motivations.
INFJs are more likely to go along with the group, try to keep everyone emotionally healthy, and focus on the emotional needs of others. INFJs have much less difficulty interacting with others, as well as less difficulty initiating relationships - such as introducing themselves to others, or welcoming them into a new group.
INTJs are more likely to take a stand for what needs to be done even if it will hurt feelings, try to keep everyone physically/financially/realistically healthy, and focus on the practical needs of others. INTJs have much less difficulty balancing their own needs with the needs of others, as well as less difficulty pulling away from others.