Actually, you'd probably find than a probing email asking what's wrong could be your best bet. That way the person has time in privacy to organize their thoughts and thus give you an honest answer. Less chance that you'd get an answer of "I'm ok", especially if it's a guy. If they don't reply and you're worried then a follow-up visit with that kind of preface would be more likely to open them up than a follow-up visit without it. At least they've had a chance to think about how they would respond to your question (ie what's bothering them), even if they didn't actually respond.
Meh, to be honest I don't know. I've thought about it. Sometimes a kick in the ass may actually be the best thing. This kind of question might be best asked of the people who have successfully dealt with INFPs on a regular basis - people married to them or friends with them etc.
Plus, I've hogged this thread enough.
Edit: Ladypinkington is married to an outstanding INFP if I remember rightly. She could probably give you some useful practical tips.