Some people are definitely harder to type than others, and less extreme dichotomies can definitely cause a problem for discerning which one is used the most.
This is a case where knowledge of the cognitive functions would come in handy.
Or perhaps you have an understanding of the differences between the NF types and the SJ types? That would help, too.
Since you're an INFJ, this idea might work for you. Sometimes I look at the negative traits of the types and compare them because I am naturally such a critical person, it's easier for me to notice their bad points than their good ones - not a good thing, I know.