I've been thinking about this topic today,and I find it's really hard to pin traits on INFJs that aren't true of other types. INFJs tend to be very polite, but other types are polite as well. INFJs tend to seek underlying patterns and don't take things at face value. Other types do the same as well. INFJs use big words, and sometimes use vague language. Other types do this as well. INFJs can be pretty oblivious and lack self awareness from time to time, but so will some other types. I guess you would look for a pattern and look for all the traits, and even if they don't have all the traits, and are missing one or two, there's a small chance they can still be an INFJ, but maybe just express themselves differently.
In a way though, it can be easy to spot, because there is that mixture of intellectual opionatedness mixed with a sort of vulnerability of wanting to be liked and/or pleasing to others, even sometimes in an online setting. That wanting to be considered likable but also wanting put your two cents and assert your opinion. Also having that authenticity while being a little bit of a chameleon. I think Fi's when they see the chameleon side of INFJs get the impression that INFJs are not authentic, when, I think a lot of us feel that we are, but the introverted intuition can be so broad and open minded that you can see something in various ways, and pick and idea or assertion that, both has personal meaning to you, but is also somewhat concurrent with popular opinion. You never become fake, because fakery tends to be something distasteful to INFJs, but you become the version of yourself that's best suited to being liked by those around you, because your mind is so broad and expansive it contains something for everyone--with a few notable exceptions, such as incredibly toxic people that you can sense out and who somewhat sicken you to the point, where you often draw the line of wanting to be pleasing.
This is how I see the INFJ psyche,and I think this is why it's so hard to spot a real or fake one, because it's something that can come with so much complexity. Maybe, however, INFJs have the right mixture of depth and populism and if you sense there isn't enough of one or the other, you could maybe start suspecting the possibility that they may not be one, but keep reasonable doubt until you really look at the person much deeper, because a lot of us think we know other users on here, or even people we interact with on a daily basis, but can we say we really know the people we interact with?