Reverse image search comes up most often as a tool for checking whether a design you're about to use is too close to something copyrighted — but it's just as useful pointed in the other direction, as a way to periodically check whether one of your own original designs has been copied and is showing up elsewhere without your permission.
Tools like Google Images' reverse search, TinEye, or Bing Visual Search all let you upload an image (or a screenshot of one) and search for visually similar matches across the web. Periodically running your own best-performing or most-distinctive designs through one of these tools is a genuinely low-effort habit that occasionally surfaces exactly this kind of issue — a design showing up on another seller's listing, sometimes even on a completely different platform than where you originally published it.
If you do find a clear match, the appropriate first step is usually a direct, polite message to the other seller, since it's genuinely possible (though not always the case) that they sourced the design from somewhere without realizing it wasn't licensed for their use, rather than deliberately stealing it — starting with a calm, factual message rather than an accusatory one tends to resolve straightforward cases faster and with less drama.
If a direct message doesn't resolve it, most platforms (Etsy, Amazon, Redbubble) have a formal takedown or reporting process specifically for intellectual property violations, generally requiring you to demonstrate you're the original creator — having your own design files, layered source files, and ideally some record of when you first published the design (a dated listing, a dated social media post) makes this process considerably smoother if it ever comes to a formal report.
This isn't something to obsessively check daily, but building it into an occasional habit — say, a quick check on your top handful of designs every couple of months — is a reasonable, low-effort way to catch the issue reasonably early if it happens, rather than discovering months later that a design has been copied and sold elsewhere for an extended period.