Those of you that experience confusion between whether or not you're an INFJ or INTJ, this is the place to look for information. That infamous T/F flip-flopping that never seems to end. You're emotional and empathetic like an INFJ, but you're also cerebral and cool-minded like an INTJ. What it all boils down to is your natural inclinations, rather than learned inclinations.
So here's ways to determine if you are INTJ or INFJ:
Ni-Fe and Ni-Te people have a directive style of communication. There's a difference between the two.
1. Te tends to be short and concise. i.e. :"Put the garbage out."
It implies "You do A".
2. Fe tends to be sweetened a bit, implying a favor. i.e. "May you put out the garbage for me?"
It implies "I'll ask you this favor in a nice way to establish a warm interaction with you."
Notice how your commanding style is. If you tend to use choice 1 more often, you use Te more than Fe. If you use choice 2 more often, asking people to do things as a favor, you use Fe more than Te.
The Stress Test
Think of the moment where you were the most stressed out. How did you respond? What is your coping mechanism afterwards?
If you are INTJ, then you will notice in the heat of stressed out moments you will respond a lot more emotionally than usual. Ni-Fi manifests itself as being like the inner child, and you will feel as if the world is out to get you, you are now vulnerable. It will be out of the ordinary for you to respond emotionally, when in most cases you remain calm and composed. After moments of stress, you will find yourself more indulgent than usual, treating yourself to more sensory related activities.
If you are INFJ, then you will notice in the heat of stressed out moments your ESTP shadow will manifest itself as being blunt and combative even towards those close to you. Tempers will flare similarly to the Ni-Fi response, but it will be much more physical... you'll feel the urge to shout or throw things, or cry. It will seem out of character for you to act this way. After the stressful situation, you'll find yourself migrating to a more logical Ni-Ti loop... a cool-down period where you will seem withdrawn from others but also like your brain has been turned on. Ti kicks in when you've exhausted all of your Fe reserves.
It all boils down to rationale. INTJs and INFJs may seem similar in that they are both logical beings, but their mode of logical thinking is quite different.
INTJ's mode of logical thinking is much more streamlined. It focuses on bringing about ideas into the physical world (Proactive thinking). It checks to see what steps one can take to achieve a desired outcome, and these steps are done in an efficient method. A goes to B, goes to C, so on and so forth. It is more empirical and searches for clear evidence to suggest that their intuition is accurate. The logic is based on usefulness and consensus of ideas in the group and coming up with a plan to achieve a desired goal.
INFJ's mode of logical thinking is more global and systematic. It resembles something like a stock-and-flow diagram, taking into account all interrelated parts of an event, person, idea, and thing. Causal loops, what effects what, who effects whom. Ti by nature searches for precision in terms, so often times INFJs will desire exact definitions and instructions to better understand what is required of them or what needs to be done. "Well, what do you mean by A? What are the implications of B? How does D effect C?" The logic is based on cause and effect, the likely consequences of actions in the present moment on the future, as well as probing for exact meaning.
INFJs and INTJs both are visionaries. The difference lies in how they go about realizing their inner dreams.
INTJs will feel comfortable being the only one supporting their vision. As long as they feel deep down that it is the right thing to do, they will go for it. They will lay out a course of steps to achieve this desired outcome.
INFJs will feel more compelled to get others to join in with their vision. There is a tendency in INFJs to wrangle in the troops to serve the common good. They practically become missionaries for their inner vision.
The Eyes Have It
The gaze of Ni-doms is typically intense. But INTJs gaze tends to have a coolness about them, assurance, confidence, a sparkle, almost like they are looking through you. INFJs gaze, while most INFJs may seem outwardly stoic, there's an ironic softness to INFJ eyes, a doleful gaze, you can tell that their thinking is directed inwards, and will have the "thousand yard stare" on occasion.
So look into the mirror and see which type of gaze you have.
This is intended to be a quick guide if you are indecisive about your type. If you fall somewhere in between the two descriptions, then the next step is to check out your cognitive profile and see which processes you use more often. Also think of what comes the most natural to you, what feels right.
If anyone wishes to add to this, feel free to do so.