Q: How can I tell if an ISFJ genuinely cares?
A: The ISFJ isn't like other personality types. Our top two functions are Si and Fe, meaning traditional memory/sentimentality and compassion for the emotions of others. We have a very high rate of compassion, with the only personality types higher being the ESFJ and ENFJ.
As a personal example, I'm in customer service. People walk up to me with their questions and their problems and their concerns every single day. I care about each and every one of them, having no preconceived notions about them at all. My job is to care.
On to the actual "tells" of a caring ISFJ:
1) If you're in serious distress, an ISFJ will touch you. Because an ISFJ is an introvert, they will not always touch people. They might experience compassion for everyone, but to know when an ISFJ cares beyond the basic call of duty is to notice if they touch you during distress. An ISFJ will hug you, rub your back, touch your arm, maybe even stroke your hair, if you're having a bad day and they truly care about you. Take me; I will touch a close coworker's arm or shoulder if they're having a bad day. It's a sign of support and empathy.
2) An ISFJ will recall details about your life and experiences. They're not focused on themselves; they're focused on you. If an ISFJ cares, they will make mental notes about you and reference back to them. Let's say your sister in Illinois is sick. The next time the ISFJ sees you, they will inquire how she's doing. An ISFJ listens to you and learns about your life if they care.
3) An ISFJ keeps in contact with you. Sometimes an ISFJs other functions are underdeveloped, such as organization in my case. My Fe is so highly developed due to personal circumstances that my Si isn't as developed as it should be. It may take them awhile to contact you, but if they put in the effort at communication, it means they like and care about you.
Some things to bear in mind:
The ISFJ has a limited amount of space in their lives for "best" friends. Too many people wears an ISFJ out. They'll have a wide range of acquaintances, and yes they care about each acquaintance, but only a few really close friends. That's just the way an ISFJ rolls. Just remember, even if an ISFJ isn't your closest friend, if they're doing all of the above: communicating, touching, listening to your experiences, then it means they care about you. It's hard sometimes to be excluded from an inner circle of friends, but just because you're not in that inner circle doesn't mean they don't care.
An ISFJ can run out of compassion. I'm sure you've all heard about Justin Bieber and Miley Cyrus making repetitive bad choices. My caring for them is minimal. So if you make repeatedly bad choices, or don't listen to the advice of an ISFJ, there will come a time when they turn off the Fe and feel absolutely nothing for you. We're scary that way because we can just switch it off if we so choose.
Overall, the ISFJ is a very loving individual. Unless you are an extremely trying individual, there's a good chance if you know an ISFJ, they care about you. They're hard-wired to do so. :)