How to Know Whether Manas Is Engaged in Exertion, Investigation, and Deliberation?

How to recognize when the mental faculty is diligently investigating and contemplating? When you feel there is always a critical phrase or a lingering doubt in your mind, it signifies that the mental faculty is engaged in thinking and investigation. When your mental attention is consistently focused on the critical phrase, and your mind is perpetually filled with questioning—even without overt acts of analysis or deliberation—yet those nagging doubts and thoughts cannot be shaken off, this indicates that the mental faculty is engaged in investigation.