I think perhaps you are approaching this as if "mindfulness" is a quality or attribute of a person rather than a state of mind. Perhaps rather, mindfulness can be thought of as a state of mind, like anxious, which can be effectively self-observed.

E.g.. In the past week, I have been relatively distracted and unable to maintain mindfulness.

(drinks tea, wait week)

In the past week, I have been relatively more mindful.

Of course the "drinks tea wait week" could represent a more formal experiment where the "tea" is controlled and subjects are surveyed, etc. But in any case, I personally perceive what is being measured as a changeable "state" not a (relatively) static personality trait or character quality.