Hmm, well if skin colors are different in your world, and they're not associated with history or social status the way they have been in ours, then those wouldn't be the obvious clues. But there are other things: facial features, hair texture, height, build, history within your world and so on.
Also, cultural norms, and religious beliefs, social class, socioeconomic status and so on. It gets complicated in fantasy worlds if they have a very different history from ours. But skin color isn't the only thing we focus on when we think of someone as being a PoC.
The real problem is the automatic white default most readers still have. If you don't say otherwise and have environmental/cultural cues people stereotypically associate with Africa, Asia, the Americas and so on, most readers will assume everyone in your world is white. And just saying "She looked like she might be from Minua," means nothing when Minua is a made-up place. But there's no reason why people who "look" Asian in your world would have rice as their primary cereal crop or eat with chopsticks or have religions that are similar to Shinto, Buddhism, or Taoism.
I'll admit, I worry about accidentally describing something in an offensive way or falling back on a tired stereotype. For instance, if I describe a character with hair that might be described as "black" hair as having her hair cut in a short, springy, mosslike crown over her entire head, will this sound like I'm exoticizing? Or is moss possibly insulting? I don't worry about describing a white kid with boing-boing curls or corkscrews, or ringlets, or stick-straight hair, or as bristles cut so short I can see his pink scalp beneath it, even if some would say that's not flattering. But I worry if I describe someone who isn't from my own cultural/racial background in a way that sounds as if I think their hair is ugly or strange.
