Other than what I've said numerous times, "Everyone is a naturist, they just don't know it yet," I have detector strategies.
When I've been single, I've been on the lookout for the criteria of potential naturist partners and cohorts. I use cultural signals to zero in. Background as hippy type when young, or currently, just how open-minded they come across, intellectual, or education level. Work/career isn't a good indicator. Therapist are generally gotten nude. Many conservatives are found in socially nude venues, so the hippy thing isn't definitive, but it does indicate better odds of finding a nudist, or potential nudist friend. Naturalist have a good chance to be one to explore nature in other ways. How one free-form dances is an indicator. Outgoing, free spirit, exhibitionist leanings make for nudity and in time, these things can tone down into a naturist and less exhibitionist. People into alternative health can often be open to it. The way someone dresses can give a clue, but just a light revealing outfit may have to do with fashion sense or comfort and not necessarily mean that more would be revealed. Unusual, non-conformist dressers can turn into a person who thinks nonconformist dress means none. Those rebellious, or thrill seeking. A nudist could be any type, but there are clues to hone in on exploiting the potential of the find.
People will get nude in specific circumstance and then find that they like it. The circumstance may broaden or not, in time. A friend may feel comfortable with another couple and get into a hot tub, or sweat, feeling safe. It could blossom into nudist slowly, or bang into it, taking to it like a duck to water.
The best indicator that I know that someone is a nudist is that they are nude.
Just some thoughts off of the top of my head.
Jbee