If you’re in a retail location, you can often bend down to look at something on a lower shelf and get a good look between the shelves.
Adjust a ball cap brim. Pull it down [look] then push it back up and wiggle it a little like you’re re-seating it. Barring that, you can sometimes ‘have a headache’ and rub your forehead with your thumb and index finger while you check things out.
Airport, you could sit in a chair, put your head back, and sit with your eyes only slits like you’re ‘resting your eyes’.
Find bright sun and ‘shade’ your eyes.
Find deep shade and just look as you like!
Watch the people on another floor via atrium.
Pretend you have something in your eye, and you’re just randomly staring in that direction because you’re distracted trying to get it out.
Lastly, just be bold and stare. You don’t know any of those people and they don’t know you. They’re more likely to think you might find them familiar as they are to think you a perv.