Almost immediately and instinctively you start noticing other hikers' footprints on the trail. You figure out the shape of each comrade's footprint and can easily ascertain who is in front of you. Having a unique shoe-print helps, but you pick it up either way.
When we all (my group of four) started this trail, we each had a different pair of shoes. Two kinds of Solomons, a pair of La Sportivas and some Asics. It made discerning one another's footprints a breeze.
With more than 700 miles between us and the Mexican border we have all moved on to at least our second pair of shoes. And now are all wearing the same model of shoe: Brooks Cascadias. By far the most popular shoe on the trail, each of us fell prey to the allure. Nearly every person I saw with them would receive the question "How are you liking your Cascadias?" as if I'd receive some sort of new answer. Unanimously, people swore by them.
Boy do I love these shoes. People were right and I couldn't be happier, mostly.
While the shoe itself feels wonderful, my whole crew now bears the same footprint, along with a ton of other hikers on the trail. At this point, I cannot tell who is in front or who is behind, only that I am on the PCT and that people are indeed in front of me.
We're part of the crowd now.