They wear American brands.
Americans wear American brands and American brands advertise themselves right there on their products, making them easy to identify anywhere in the world. A jacket isn’t just a jacket—it’s a North Face jacket. Brands like Old Navy, Ralph Lauren, American Eagle, Hollister, Gap, and so many more very American brands all sell products with their company names displayed all over them.
Additionally, wearing T-shirts showcasing various events you took part in or places you visited will do the trick in the same way. Think: the one you have from that 10k you completed, or that fundraising event you attended, or that museum you visited.