It should be a relaxed and casual observation. Take advantage of times to scan that makes sense, like about to cross a street etc. Use reflections from store/car windows. It shouldn't be a paranoid scanning, just laid back observation.
Pay particular attention in "transitional" areas. Areas that are between one relatively safe location (say, inside your car) and another (inside the building you are visiting). Transitional areas are typically parking lots, alleys/walkways or any other publicly accessable area with less people around and more cover to commit a crime.
Think like a criminal, where would you hang out to mug someone? Pay particular attention to those areas! For example, as you approach a dumpster, give it some extra room (don't walk right beside it) and scan behind it as you pass. Same with rounding any corner etc. Give it a little extra room (reaction time) and visually clear it as you go by.
Nobody will notice you are doing these things except the criminals (because they see that you see them and thus don't target you) and maybe cops. Everyone else has their head in a smartphone. "Walking ATMs"
Finally, don't be afraid/embarrassed to just change directions and leave the area if you get a funny feeling about someone. Cross the street, act like you forgot something and turn around, don't get in the elevator with the creep! "But he'll think I'm racist if I don't!" OK, so willing trap yourself in a small metal box with someone who puts you in fear for your safety because of what a stranger
might think if they
aren't a rapist, makes sense...
(A lot of women get victimized due to thinking and behaviors like this...men get victimized because they can't control their egos)