First off, I have no idea how anyone but me (including other INFPs) perceives anyone else.
My impressions of people I meet in real life are basically two-fold. Part one is physical, so I can identify you. Part two is about your emotional stability and how it relates to mine and to any other people around us. (I guess if there’s a part 3, it would be resonance or recognition, a spark – something like that.)
I need a specific level of chaos in my life, and I will subconsciously alter my behaviour with each interaction I have so that balance will be maintained. This means that I will be the epitome of the INFP peacemaker stereotype in a room running high with emotion, but it also means I’ll be the weirdo in a room of stoics. (And if I get punchy enough to operate out of my shadow functions, I can and will escalate like a boss.) If someone else is already filling these roles, I’ll probably just observe quietly and/or cheer them on – it’s less emotionally draining than being a counterweight to the people around me.
As you might imagine, it makes it incredibly weird (both for me and others) when people I’ve acclimated to in different situations get together and I have to reconfigure my behaviour to match the new group dynamic. I am frequently blindsided by my own social flailing in this respect.
You’d think that since we share Fi, ISFPs would have all of this in common with us, but I don’t think it works that way. I think they are sensitive to the same emotional balance, but because of Se, which demands physical experience, ISFPs are more inclined to risk and less inclined to change themselves to avoid conflict (physically remove themselves, though, yes, which is more of a plan B for us). They want to do all the things and feel all the strong emotions, and Ne would rather experience most of that in the safety of our own heads.
But okay, as for typing ISFP vs. INFP, which is probably closer to what you were asking about, I look for 1. presentation, 2. concrete vs. theoretical answers, 3. emotional resonance.
1. S types are typically better dressed, take better care of hair and make-up, wear more and more varied accessories. S types also like their presentation to mirror their perception of themselves (or vice versa). N types usually learn to dress well if it’s an expectation of their work or if it’s an area of specific interest to them, but don’t necessarily bother otherwise. They don’t always represent themselves well through their image, either (or mean to). INFPs and ISFPs both wear clothing emotionally, but ISFPs care more about the finished look (it shows).
2. Differentiating between N and S on ISFPs and INFPs can get tricky, because introverts like to hide what we’re thinking. We have pretty equivalent levels of awkwardness – it can take a while to translate thoughts from Fi into words, and things get lost or disordered in the translation. Trying to clamp down on revealing too much Fi or confusing people with Ne adds another level of frustration to conversation. On the other hand, both ISFPs and INFPs can also be very articulate – usually at this point it’s not whether they’re S or N, but whether they’re I or E that’s tricky. Generally speaking, I lean towards S, unless I hear someone specifically say something N-ish.
3. Resonance… I don’t relate to ISFPs in ways that I understand and recognize, having been around a lot of them. It makes it easy to see when they’re not my type. I have a harder time acknowledging other INFPs since I don’t always relate to them in ways I don’t recognize, but there’s usually something there that makes me wonder.
People have a wide range of behaviours within type, so sometimes one ISFP will strike me exactly like another ISFP, but more often I don’t know people who act the same way and have to narrow them down until I’m at the point of “what else can they be?” People can act uncharacteristically or just differently for a lot of different reasons. To me, at least, body language, clothing choices and language choices – speed and patter of thoughts and words – are a lot more consistent as far as seeing patterns between types than behaviour.
Yikes, this is long. Well… I kind of tangent-ed horribly, but I hope you still got a somewhat satisfying answer out of that.