1. What are some clear cut differences that you have noticed between each of these types?
I'm going to compare them to INFPs, since I am one, and that's easier for me.
ISFJs are very self-sacrificing and in nurturing in very tangible ways (ie. they DO things for people). They seem to take on a lot of responsibility. They accomplish quite a bit, as they have a lot of energy for introverts. Usually have a strong sense of morality, that is very tied to the community standards. Can be very creative and have a good eye for space/color, especially with hands-on activities that have a practical purpose (cooking, gardening, decorating, fashion). Tend to do things in "good taste", which don't offend, but may not be the most fresh to the eye.
INFPs, in contrast, are a little more self-absorbed, disconnected from the real world (accomplish less tangibly), may avoid responsibility for
tasks (but readily take it on for
ideas), more likely to "heal", which isn't always so nicey-nice (ie. sometimes it means ripping off the metaphorical bandage and poking around), but it's because we are doing things with a view to the long-term. Their strong sense of morality is internal, meaning it can be worlds away from the community so they tend to seem more rebellious/autonomous
compared to an ISFJ. Tend to lean towards the arts, be it literature or visual arts, but unlike the ISFJ, may create just to create, with no practical purpose. Might seem more offbeat and "original", but not as even with their taste (may veer into the bizarre and garish).
ISFPs tend to differ from INFPs in being less interested in intellectual things (not less intelligent though). ISFPs will often still enjoy abstract things though. Fi is abstract in itself, plus they have tertiary Ni, and SPs in general may like things that are a bit rebellious or unusual. The bookishness of the INFP often makes them seem a bit nerdier actually. You might just ask if they have a library card and then watch their face: the ISFP may smirk, and the INFP's eyes may light up
2. What are the similarities?
They're all introverts and feelers, so if you see someone who is quiet and generally emotional, they might get tagged with any of these. However, I'd bet the ISFJ comes off the most emotional/warm externally.
Jung describes Fi-doms as aloof, but I'm not sure how much the Ne and Se offset that. When being somewhat "outgoing", ISFPs seem rather warm to me and more connected to the moment, and outgoing INFPs seem rather wacky. Outgoing ISFJs can be almost boisterous. They definitely seem the most "extroverted" at times, although they still may seem very gentle and quiet at other times.
3. What are traits that make you think ISFJ (or ISFP / INFP) right off the bat?
Stuff that makes me think ISFJ: someone who is very giving by doing things, has a home/appearance that is well put-together (may or may not be fashionable), seems warm but quiet, has a serious reverence for "rules" and order, has trouble saying "no" or standing up for their own needs, speaks in direct terms though, and likes to talk about people and know about their lives.
For INFP, I usually go with: someone who is kind of weird, into some form of art or philosophy, like to discuss theories/ideas or emotional issues, but not people's activities, often have some unusual moral stance, seem aloof and cool on the surface, but you realize they are sensitive and emotional after getting to know them, tends to use passive speech ("I would like" as opposed to "I want"), but may be more open to being the voice of dissent.
For ISFP: also have a "different" demeanor, may be construed as anything from aloof to strange, often into the arts, likes physical activities and hands-on things more than an INFP does (ie. body boarding, hiking, dancing, etc), may be more prolific artists because of that, usually very connected to nature/animals, have strong feelings about things also, but less universal/idealistic than an INFP (who may seem to over-complicate issues), may be more demonstrative of positive emotion (if only because more connected to reality), less articulate in writing/speech than an INFP, and as I said above, less interest in intellectual things.