(a)alert, watchfulengaged in or accustomed to close observation“caught by a couple of alert cops”, “alert enough to spot the opportunity when it came”, “constantly alert and vigilant, like a sentinel on duty”
(s)argus-eyed, open-eyed, vigilant, wakeful, watchfulcarefully observant or attentive; on the lookout for possible danger“a policy of open-eyed awareness”, “the vigilant eye of the town watch”, “there was a watchful dignity in the room”, “a watchful parent with a toddler in tow”