on the lookout

adjective as in alert

adjective as in attentive

adjective as in cautious

adjective as in guarded

adjective as in on one's guard

adjective as in on the ball

adjective as in open-eyed

adjective as in politic

adjective as in unsleeping

adjective as in vigilant

adjective as in wakeful

adjective as in wary

adjective as in wide-awake

adverb as in on guard