Frank J. Wilstach, comp. A Dictionary of Similes. 1916.

Watchful

Watchful as a sentinel.
—Anonymous

Watchful as the eye of a bird.
—Anonymous

Watchful as a bellman.
—Beaumont and Fletcher

Watchful as a spider sits in his web.
—Edward Bulwer-Lytton

Watchful as the step of a mother by the couch of her sick child.
—Edward Bulwer-Lytton

Watching every glance of him like a British house-dog that will not be taken in with suspicious travelers if he can help it.
—Thomas Carlyle

Watchful like owls awake.
—Gilbert K. Chesterton

He was trying to see, with that watchful manner of a seaman who stares into the wind’s eye, as if into the eye of an adversary.
—Joseph Conrad

Watchful as spirits.
—Abraham Cowley

Watchful as when fowlers their game will spring.
—Thomas Otway

Watchful, as a leopard is.
—Dante Gabriel Rossetti

Watchful as a wheeling eagle.
—William Wordsworth