Google Images, to see if there's an author photo on the web anywhere. And yes, the categories box at the bottom of the Wikipedia page, if the bio doesn't say outright.
The thing that I find frustrating is that you only ever get confirmation that someone is POC; you never get confirmation that an author isn't. Yeah, sure the photo may be kinda white-looking, but that doesn't rule out a Native author, nor a Latin@ author, nor Middle Eastern or North African. So you poke at the internets for a while, and eventually decide, "No, it's not just that I've hit five pages in a row where no one saw fit to mention the author's ethnic heritage; they're not mentioning it because she's white and the people writing these pages think that means there's nothing to mention."
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The thing that I find frustrating is that you only ever get confirmation that someone is POC; you never get confirmation that an author isn't. Yeah, sure the photo may be kinda white-looking, but that doesn't rule out a Native author, nor a Latin@ author, nor Middle Eastern or North African. So you poke at the internets for a while, and eventually decide, "No, it's not just that I've hit five pages in a row where no one saw fit to mention the author's ethnic heritage; they're not mentioning it because she's white and the people writing these pages think that means there's nothing to mention."