Most Feeling types will tell you they feel they understand people well or have an interest in doing so, but to me, F-dom with aux iNtuition (INFPs & ENFJs) are more preoccupied with refining their understanding to be used on a larger scale. With F-dom, the primary focus of their mind is what things mean & what is their significance in in terms of people, and then iNtuition aids in seeing what is not immediately apparent on the surface. From an INFP perspective, you form general concepts to work off of, and then you can make some pretty dead on inferences about how people are & why they are that way. This can occasionally result in being dead wrong when projection is involved (and projection is more often an issue of Pe for xxFPs than Fi, IMO), but that doesn't mean it's a total crapshoot. You're seeking to make sense of people in emotional terms (which is not logical, but it has a sort of consistency to it still), so it's not just trusting a gut feeling (which may be more true of P-dom).
What I notice with xSFPs is a heavier reliance on first-hand experience. Projection occurs when their experience is limited & they use this limited knowledge to draw a conclusion & don't consider other possibilities (whereas a Ne type may invent possibilities that don't exist). However, they also work with more concrete, factual information, which has obvious advantages. I see that with SFs, period. My ISFJ mom can recall physical things about people that lead her to the same conclusion I get, but I can't connect it to anything solid. I just "know", and I make sense of this knowing using my general understanding of how people work.
Oh, and I think I should add that xxFPs are probably more preoccupied with noting and annoyed when they suspect that someone is being "fake", just due to the nature of their own mindset.