Is it waiting or loitering? Learn to identify intentional loitering, especially around transit stations and their restricted areas.
Trust your gut. Learn to sense when a person’s behavior just isn’t right in a certain place, or at a certain time.
Learn to be alert for someone using binoculars, taking photographs or otherwise documenting a transit structure.
Step up your game. Know the special times and places when you need to ramp up attention to your surroundings.