Ah, so you're asking about trying to distinguish between INFJ and ISFJ for someone? Your method seems a bit roundabout as it's usually easiest to type someone through their use of the primary and secondary functions. So trying to find determine if a person is INFJ or ISFJ, you should look at whether the person engages Ni-Fe or Si-Fe more.
If it's still unclear from that, or if the person is in a very unhealthy psychological state (Being stuck in a primary-tertiary loop isn't unhealthy but isn't necessarily healthy either and trying to constantly engage one's inferior is always bad news) then learning about the person's stress points would be illustrative in determining one's inferior function.
If it's still unclear, then trying to determine a person's primary-tertiary loop would be useful. Ni-Ti loops in an unhealthy way would manifest itself by a constant interpretation and analyzation of symbols/hidden meaning to the point where it causes paralysis in the person. In a more healthy way, Ni-Ti loops would be a deeper understanding what's stated on the surface.
With regards to engaging the inferior for the INFJ: The INFJ inferior is Se which much like an Ne inferior is all-or-nothing. Except Se-inferior in the INFJ and INTJ comes about in engaging themselves physically in excess or completely depriving oneself of engaging themselves physically. Either way, an INxJ should not try to engage their Se if they're uncomfortable with it.