observant

Alert and paying close attention; watchful.

The observant police officer noticed that my tax disk was out-of-date.

Diligently attentive in observing a law, custom, duty or principle; regardful; mindful.

I was normally observant of the local parking restrictions.

We are told how observant Alexander was of his master Aristotle.

Practicing a religion.

She is an observant Catholic who goes to church regularly.

A member of a Franciscan order that strictly observes the rules of St. Francis.