on the alert
Very attentive, or vigilant
Wild animals have to be constantly on the alert for predators.
Synonyms: on one's guard, on one's toes
One's prime is elusive. You little girls, when you grow up, must be on the alert to recognise your prime at whatever time of your life it may occur. You must then live it to the full. Muriel Spark
These so-called governments are in reality only great bands of robbers and murderers, organized, disciplined, and constantly on the alert. Lysander Spooner
Be on the alert to recognize your prime at whatever time of your life it may occur. Muriel Spark
Someone who's on top of the world isn't much of an observer: happy people are poor psychologists. But someone who's troubled about something is on the alert. The perceived threat sharpens his senses - he takes in more than he usually does. Stefan Zweig
As Huxley remarked in Brave New World Revisited, the civil libertarians and rationalists who are ever on the alert to oppose tyranny "failed to take into account man's almost infinite appetite for distractions." Source: Internet
Due to the flight's routine delay, the pilot and crew were notified of the previous hijackings and were told to be on the alert, though within two minutes Jarrah had stormed the cockpit leaving the pilots dead or injured. Source: Internet