For me it's not so much something that comes from having some technique or explicit knowledge on the differences, it just improves with experience being around people and the more I familiarize myself with function theory. I wish there was a trick to it, but I don't think there is. For INFPs though, work on objective analysis. I know I can get caught up in my similarities to a person that I miss what makes us different, and especially the other way around. Like it took a long time to confirm my sister as an INFP because we are so different. Put Fi to the side for awhile and get Ne and maybe even Te in on the typing action as much as possible.