1. How can you tell which cognitive function is being used?
2. Can more than one cognitive function be used at once in a given situation?
3. How possible is it to use introverted functions in an extrovert setting and vice versa?
4. Or is it that all functions are running constantly, sometimes one takes over depending on the situation?
1. Personal experience
2. Yes. I believe people always used CF as a pair(ESFP is Se-Fi, Te-Ni for example, but not Se-Te or Se-Ni). This CF pair must be two CF which can support each other following the rule: I-E and T/F-N/S.
3. The 4 CFs work as the master-slave model. The 3 later CFs who are slaves must serve the dominant one who is master CF. In other words, an ESFP use his/her Fi in the Se way, and it's different from an ISFP use his/her Fi. We still can discern the Fi from other CFs though.
4. Yes. They are running constantly, but most of the time we can only see two first CF, the rest are two weak to be recognized, till in late of our life.
In short, I would say our 4 CFs can be recognized by two attribute:
- Intensity(or other better replaced words, sorry my English grammar suck): measure how frequently and strongly we use our CFs. The strongest is dominant CF and the weakest is inferior function.
- The role of each CF. It's not simply the intensity of the CF, but their role also. The 1st and 2nd is the most important CF since they are strongest and we can tell what type of a person by recognizing them. All of CFs work for the dominant as its slaves. We use other weak functions in the dominant function way.
By saying "personal experience", I mean if you want to apply the MBTI CFs into real life, you must be really in it. You need to understand the CF concept first, then observer other people around you. You use your absorbing information function(either S or N) to "eat" the information about your object(people I mean). And finally trying to figure out what type of them are. You must be subjective, don't mind to be subjective and don't believe in Ti-objective bullshit if you are an INTP. Feel free to be subjective because it's our personal subjective opinion after all.
From time to time, depend on your ability, your effort you put in it you will have experience( not necessary Si I mean) and skills. It's subjective, it's biased and it's may wrong but who care? However what I really get from my experience about recognizing other CFs is about understanding myself. I'm not F type after all why I have to care about other, bullshit. The more we understand other the more we actually tend to understand ourselves, to be change, to be adapt and to be survived.
P/S: Honestly I don't trust those MBTI youtube videos. Watching them for fun, fine, but for education or learning something I don't think so. Because they are their subjective opinions actually, why we have to take them while we already have own one? Trusting the core and trusting yourself are better.