Personal trainers, dieticians, high motivation given that their job relies on them staying hot, plus you might find one or two have the advantage of cosmetic surgeons and airbrushing...
Plus genetics and general body frame play a massive role.
I don't see how you can tell because it's all proprtional, I have eyes but I don't carry a measuring tape? A curvy 28 inch waisted woman will look like she has a smaller waist because it's smaller compared to their hips and bust, whereas a more gamine althletic build with a naturally smaller waist might not look all that small if they're straight up and down, it's practically the same width as the rest of their body.
Also, studies have shown that men are most attracted to a waist to hip ratio, rather than the measurement. Like the most attractive women in the world like marylin monroe, kim kardashian and cheryl cole all had the same ratio of around 0.6/0.7 despite having very different body types, so the measurement is somewhat irrelevent...