So, we can never “see” mano sankhara, but we can see the effect of it.
Let’s say that an unwholesome conscious thought is present, such as a thought of violence toward someone; then we know that there is unwholesome mano sankhara behind it.
On the other hand, if we observe a wholesome thought in ourselves, such as a thought of generosity, then we know that wholesome mano sankhara is happening.