Short answer: When they are interested in your ordinary life.
Longer answer:
I think that ISFJs are usually rather conventional when expressing themselves. They don't do that much flirting, or if they do, it's very polite and restrained. They'll probably compliment you for something. Or just smile at you. They are also interested in talking with you about any topic. They'll probably won't be very assertive with their emotions, but they like to socialize with you. So I guess ISFJ interest would look a lot like genuine admiration.
Habba nailed it.
As an ISFJ guy, when I'm interested in a women I take a very polite, warm, friendly and confident approach to her and I try to let her know in small but obvious ways that I find her very interesting. For example, I'll show a much higher than normal interest in your personal, mundane, day-to-day life, I'll pepper you with compliments and I'll say things like, "Well, I just think your very interesting" or "Your ex didn't do/say [X] to you? That's his loss/That was very stupid of him" and things like that.
When I decide to finally go somewhere with it, I'm very formal and polite about it. I'll kindly ask you out to a social function or situation or something like, something "old fashioned" and gentlemanly like. Since taking up ballroom dancing, I've found it to be an excellent icebreaker when talking to women. I've taken some of my female friends out dancing and even the most reserved of them get into it very quickly :nod:
By and large though, what I do depends on the signals I get from her. If she doesn't appear romantically interested then I'll quietly back off. Otherwise, I'll move forward. What can be very difficult for me though is that I am horrible at receiving signals from women and unless they are obvious about their own interest, will almost always construe their actions as being friendly...but solely platonic.
This of course doesn't all take into account circumstances in which I'm infatuated with her or not.
ISFJs I think aren't a very dominating personality type so often
we are the ones who are pursued rather than doing the pursuing. When I find out that a woman is pursuing me and I'm interested in her, I try to make it very VERY easy for her to catch me.
I have to say, I much prefer to be pursued though. Women are often (but not always) such sensitive creatures when it comes to their feelings and it's endearing to watch them take the initiative and try to show interest in a guy
