I think there's good stuff in dreamer's list.
My own take is this (some of it already said)
INFp-Fe is more likely to be mistaken for an ISFp; INFp-Ni, for an INTp
INFp-Fe will be more concerned about maintaining positive emotions, so more "charming", INFp-Ni can appear more often "sad" and ironic
INFp-Ni is more likely to assume an "angry teenager", "badass" personality on occasion than the INFp-Fe
INFp-Fe will be more concerned with peaceful co-existence within a group; INFp-Ni will be more likely to try to lead, subvert or leave the group
INFp-Fe is more likely to freely discuss ideas in an easy-going way; INFp-Ni can appear more rigid about ideas they care about
INFp-Fe makes more of a "diplomatic" impression, INFp-Ni more of a "lonely idealist" impression.
These differences are a bit exaggerated to make the point; of course they remain the same type and such differences may be subtle to spot.