Anyone interested in personality typing knows that it isn't always easy to pinpoint someone else's type. Introverts are especially hard to type unless you know the person well. INFJs are no exception. They are complex individuals with a strong value system. Typing an INFJ involves looking for specific clues (see Resources below).
Watch their body language. INFJs are quiet. In social situations, the INFJ is often uncomfortable. They prefer one-on-one contact to group interaction. Approval or disapproval can often be seen in their facial expressions. An INFJ walks erect and with purpose and they appear to be focused on where they are going. The INFJ also tends to be uncoordinated and is likely to bump into a table or trip on a step.
Listen for verbal clues. An INFJ uses a lot of figurative language with deeper meaning. Because of the unconscious nature of their dominant function, Introverted Intuition (Ni), the INFJ will often seem to ramble as they search for the right words. Parasitic words such as "like" or "um" litter their speech as they attempt to express themselves coherently.
Get in touch with their feelings. INFJs use Extraverted Feeling (Fe) to express themselves. Using Fe, an INFJ will use language to express their values. It drives them to go to great lengths to promote harmony. Extraverted Feeling motivates the INFJ to help others. Fe also causes an INFJ to do things because it is appropriate. They care how they are perceived by others.
Check out what they're wearing. An INFJ will choose appearance over function. Their outfits are usually conservative and coordinated. INFJs usually consider expensive clothing to be a luxury as opposed to a necessity.
Study cognitive processes to get a better idea of how to recognize the Introverted Intuition and Extraverted Feeling present in an INFJ.
Take care not to stereotype any type. Typing others is hard work and using individual letters such as E/I, N/S, T/F, and J/P can lead to mistyping. Be aware that INFJs often look like INFPs, INTJs and ISFJs.