My ex-wife is an INFJ and one of her best friends is an INTJ. For a while, my ex was sure her friend was also an INFJ. However, the biggest blaring difference is that her INTJ friend is a walking encyclopedia of facts and also loves to analyze these facts while my ex has a hard time remembering a phone number for a few seconds (a complaint I’ve heard from other INFJs).
Her friend uses very precise language while my ex tends to make up words on the fly and slightly resents having to use language at all. She would prefer people just “get” each other without the burden of words. This approach would make sense if everyone were as good at reading people as she is. She’s ***!!!amazing!!!*** at it.
This is just personal observation, but most of the INTJ types I know have a particular mannerism where they cock their head to one side and look up in the air to that same side (I think it’s usually the left) with a quizzical look on their face while pondering a question that deserves their thought. I’ve never seen an INFJ do this. Her friend does this all the time.
Long term planning is another difference. While INFJs tend to be efficient, INTJs tend to be more detailed about how they will get something done. They’ll have plans and backup plans, and more backup plans. They’ll see the steps they need to take to achieve the end goal. I think for INFJs, it’s usually not that consciously logical.
INTJs tend to apply their intuition to systems and mechanisms. They can have amazing insight into the workings of things, while INFJs have amazing insights into the workings of people. Another INTJ friend once figured out how something went wrong with a multi-million dollar computer program. She actually experienced a vision about how the whole thing worked, and where the problem was. She told her supervisor where the problem was and she was right. Everyone was amazed and asked her how she knew. Her reply was, “If I told you, you wouldn’t believe me.” My ex is like this with people: She’ll just see what’s going on with them and where the problem is; it’s a bit like x-ray vision.
My ex, even for an INFJ, has a fairly extreme version of intuition, but what makes her an INFJ instead of an INTJ is that the focus tends to be on people rather than mechanisms.