Is it permissible to ask Muslims—upon meeting them—about the state of their faith or the state of their hearts, given that some say this is from the Unseen, which should not be inquired about?

It is prescribed for a Muslim to enjoin one another to the truth and to patience with it, and to call to it; in accordance with the Almighty's saying: (By the Aṣr [time]. Indeed, mankind is in loss, Except for those who have believed and done righteous deeds and enjoined one another to truth and enjoined one another to patience.)

The Companions, when they met, would not part until one of them recited Surah Al-Aṣr.

As for asking about the state of hearts and faith, it is feared that this may be a means of self-praise or showing off (riyaa'), and it may involve accusing the person being asked of negligence, or compelling him to embellish his answer. Therefore, the Salaf were not known to ask about such matters.

The famous hadith attributed to Al-Harith ibn Malik regarding this is weak and not authentic.

Therefore, one should refrain from such questions and suffice with asking about the general well-being of a brother, such as: "How are you?" or "How did you wake up this morning?" as this has been reported from the Prophet, peace and blessings be upon him, such as his question to the old woman Juthama Al-Muzaniyyah: "How are you all? How are you after us?"