The strictest possible interpretation would allow for this question to detect a monster using the Regeneration ability, the variable length counted ability that restores the user to full body. During that count is the only circumstance where a creature is actively regenerating; a person does not actually use the Regeneration ritual until they would otherwise dissipate. Since the ritual action is accompanied by the verbal "I Regenerate 1...(etc.)", and takes the same three seconds asking the Healing Arts question does, you would never actually learn useful information about the ritual from asking this question.
It is my personal opinion that this question is something of a left-over rule. The only instance of the word "regenerating" in the entire rule book is in the Healing Arts description where this question is listed. As a minimally useful esoteric rule I would generally leave it up to local marshals to determine when it was applicable; for example, I would be comfortable allowing the use of this question to determine if a monster was going to Revive even though that is not strictly supported, at least on the rare occasion that a monster with Revive was a valid target for Healing Arts.
